Appendix: Written Casual Japanese

付録：書き言葉のくだけた表現

Written casual Japanese is not spoken Japanese written down. It is a distinct register with its own conventions, born from a specific constraint: writing lacks the prosody, intonation, pacing, and facial cues that spoken Japanese relies on to convey tone. To compensate, writers in LINE messages, SNS posts, manga speech bubbles, and internet forums have developed orthographic, lexical, and structural tools that have no spoken equivalent — or that function differently in writing than their spoken counterparts do in conversation. A learner who can follow casual spoken Japanese may still struggle with a LINE thread or a manga page, because the signals are medium-specific. This appendix covers fifteen of the most important written-casual patterns, organized into four groups: orthographic emphasis markers, reaction and evaluation markers, particle and structural conventions adapted for text, and the stylized speech registers of manga. Each entry is shorter than those in the main body, but follows the same template. Cross-references to main-body entries are provided where a written pattern derives from or parallels a spoken one.

A. Orthographic Emphasis Markers

A.1 〜ー / 〜〜（長音）

← 話し言葉の対応: 母音の引き伸ばし（イントネーション） — vowel lengthening in speech

Formula: [final kana of word] + ー (or repeated kana: 〜〜)

Register: ★★★ core Medium: written — LINE/text, written — manga, written — SNS

Gap Note

Genki and most N4 materials teach katakana long-vowel marks (コーヒー) but never address the use of ー after hiragana or as an expressive device. In speech, a speaker stretches a vowel to show excitement, exasperation, or warmth — the listener hears the lengthening directly. In writing, the long-vowel mark or repeated characters serve the same purpose, but learners who have only encountered ー in katakana loanwords do not recognize ありがとー or すごーい as emphatic variants of words they already know. The result is a comprehension stall on some of the most common written forms in casual digital Japanese.

How the transformation works

The writer appends ー (or repeats the final kana) to the last syllable of a word to represent the vocal stretch that would occur in speech. This is purely expressive — it adds emotional color without changing meaning. The number of repeated marks loosely correlates with intensity: すごい → すごーい → すごーーい.

Examples

[context: LINE message / close female friends / expressing thanks] ありがとー！助かるー Thanks! That's such a help!

[context: SNS post / reacting to a photo of food] おいしそーー That looks sooo good

[context: manga panel / teenage girl / expressing surprise] えーーうそー Whaaaat, no waaay

[context: LINE message / male college student to friend / expressing tiredness] 今日まじ疲《つか》れたー I'm seriously so tired today

Variations

〜い → 〜ぃ（小書き） Formula: [final い of i-adj] → ぃ [context: LINE / young female speaker / cute emphasis] かわいぃ〜 So cuuute

A.2 〜っ（文末）

← 話し言葉の対応: 声門閉鎖《せいもんへいさ》による強調 — glottal stop for emphasis in speech

Formula: [word/sentence] + っ (terminal small つ)

Register: ★★★ core Medium: written — LINE/text, written — manga

Gap Note

Terminal small っ does not appear in any standard N4 textbook because it is not a grammatical form — it is an orthographic convention that represents a sharp vocal cutoff. Learners encounter 嘘っ！ or やばっ in manga and text messages and either misread the っ as part of a て-form contraction or simply cannot parse it. The pattern is extremely frequent in manga, where it is the primary written signal for shock, sudden realization, or emphasis.

How the transformation works

In speech, a speaker cuts a word short with a glottal stop to express surprise or sharp emphasis — the sound simply stops. In writing, the small っ at the end of a word represents this cutoff. It signals that the speaker's emotion interrupted the word before it could finish naturally. No additional syllable follows.

Examples

[context: manga panel / character realizing something / shock] 嘘《うそ》っ！ No way!

[context: LINE message / reacting to unexpected news] まじっ Seriously?!

[context: manga / female character / sharp surprise] やばっ、遅刻《ちこく》する Oh crap, I'm gonna be late

[context: text message / male speaker / sudden realization] あっ、忘《わす》れてた Oh! I forgot

A.3 〜。。。/ 〜…

← 話し言葉の対応: 言い淀《よど》み / trailing intonation — spoken trail-off

Formula: [sentence fragment] + 。。。 / …

Register: ★★★ core Medium: written — LINE/text, written — manga, written — SNS

Gap Note

Genki and standard N4 materials do not address ellipsis as a communicative device. Learners know 。 as a sentence-ender but have no framework for reading 。。。 or … as deliberate incompleteness. In spoken Japanese, trailing intonation and silence perform this function — the listener hears the speaker's voice fade. In writing, the ellipsis must do all of that work visually. Learners who encounter それは。。。 or ちょっと… in LINE often assume the writer made an error or that a message was cut off, rather than recognizing a deliberate signal of hesitation, sadness, or implied meaning.

How the transformation works

The writer places 。。。 (repeated periods) or … (ellipsis character) at the end of an incomplete sentence to signal that something is being left unsaid. The unsaid portion may be an emotion the writer cannot articulate, a negative response softened by omission, or a mood the writer wants the reader to infer. Japanese convention often uses three periods (。。。) rather than the Western ellipsis mark, though both appear.

Examples

[context: LINE / responding to a difficult question / hedging a refusal] うーん、それはちょっと。。。 Hmm, that's a bit...

[context: manga / character conflicted / internal monologue] 本当《ほんとう》にこれでいいのかな… Is this really okay, I wonder...

[context: LINE / friend sharing bad news / trailing sadness] 今日の試験《しけん》、だめだった。。。 Today's exam... it didn't go well...

[context: SNS post / ambiguous emotional state] なんか最近《さいきん》よく分からない… Lately I just don't really know...

Dialogue

[written — LINE / two female college friends / A sharing disappointment]

A: 今日のバイトの面接《めんせつ》。。。 [Today's part-time job interview...]

B: えっ、どうだった？ [Wait, how did it go?]

A: うん。。。まあ。。。 [Yeah... well...]

B: だめだったの？ [It didn't go well?]

A: 多分《たぶん》ね。。。また探《さが》す [Probably not... I'll look again]

B. Reaction and Evaluation Markers

B.1 笑 / w / 草

← 話し言葉の対応: [written-only form]

Formula: [sentence] + 笑 / w / ww / www / 草

Register: ★★★ core · youth Medium: written — SNS, written — forums, written — LINE/text

Gap Note

No standard textbook covers laughter notation, yet it appears in virtually every casual online exchange. Learners encounter 笑, w, and 草 and either skip them or attempt dictionary lookups that return irrelevant results. The critical comprehension issue is that these are not interchangeable — they signal different levels of formality, different generational associations, and different degrees of irony. Using 笑 on a 5ch thread reads differently from using 草, and a learner who cannot distinguish them misses social register information in every online text they read.

How the transformation works

笑 is an abbreviation of 笑い (laughter) and is the most neutral and widely understood marker. w derives from 笑う (warau) — the initial letter became shorthand on early internet forums and multiplied for intensity (www, wwww). 草 (grass) is a visual metaphor: a row of w's (wwwww) looks like grass growing, so 草 became slang for heavy laughter, particularly on platforms like ニコニコ動画《どうが》 and 5ch. 草 carries more ironic or detached energy than 笑.

Examples

[context: LINE / casual exchange between friends] それ面白《おもしろ》すぎる笑 That's way too funny lol

[context: internet forum / reacting to a story] まじで意味《いみ》わからんwww I seriously don't understand lmao

[context: SNS / reacting to own misfortune with ironic distance] 電車《でんしゃ》乗《の》り間違《まちが》えた草 Got on the wrong train lol

[context: LINE / younger speaker / light reaction] ウケるw That's hilarious lol

Variations

大草原《だいそうげん》 Formula: [sentence] + 大草原 [context: forum / extreme laughter / ironic] 大草原不可避《ふかひ》 Can't help but die laughing (lit. "vast grasslands unavoidable")

B.2 ワロタ / ウケる

← 話し言葉の対応: ウケる is also spoken; ワロタ is written-primary

Formula: [standalone reaction] or [sentence] + ウケる / ワロタ

Register: ★★ common · youth Medium: written — SNS, written — LINE/text, written — forums

Gap Note

Learners who encounter ウケる may try to parse it as the verb 受ける (to receive) and become confused when it appears as a standalone reaction to something funny. ワロタ is even more opaque — it is a past-tense form of 笑う in a dialectal/internet-slang conjugation (笑った → ワロタ) that no textbook covers. The comprehension gap is compounded by generational shifts: ワロタ peaked in the 2000s-2010s internet culture and now reads as slightly dated or deliberately retro, while ウケる remains current. A learner who cannot read these generational signals misses a layer of social information.

How the transformation works

ウケる comes from 受ける in its extended sense of "to land" (as in a joke landing). In casual speech and writing, it functions as a one-word reaction meaning "that's hilarious." ワロタ is an internet-era conjugation — 笑う treated as if it were a 五段《ごだん》 verb in a Kansai-inflected past tense, then written in katakana for visual impact. Both function as standalone reactions or sentence-final tags.

Examples

[context: LINE reply / reacting to a friend's story] ウケるんだけど That's hilarious though

[context: SNS quote retweet / reacting to a viral post] ワロタ LOL (I died)

[context: LINE / group chat / reacting to a photo] この顔《かお》まじウケる This face is seriously hilarious

B.3 やばい（評価語）

← 話し言葉の対応: やばい is spoken and written; written use shows broader semantic range

← Spoken base: this pattern is pervasive in speech but its written use on SNS extends the bleaching further

Formula: やばい / やば / やばっ / やばすぎ (standalone or pre-noun)

Register: ★★★ core · youth Medium: written — LINE/text, written — SNS, written — manga

Gap Note

Some newer textbooks mention やばい as slang for "dangerous" or "bad," which was its original meaning. What they fail to address is the near-total semantic bleaching that has occurred in casual usage: やばい now functions as a general-purpose intensifier meaning "extreme" in any direction — delicious food is やばい, a beautiful sunset is やばい, a terrible exam score is やばい, a funny video is やばい. Learners who only know the "dangerous" sense will misread positive uses entirely. The written forms やば and やばっ (see A.2) add further confusion, as they are truncated forms that appear constantly in text but are not in any dictionary.

How the transformation works

やばい has undergone semantic bleaching: its core meaning shifted from "dangerous/risky" to "extreme/intense" without polarity. Context alone determines whether the evaluation is positive or negative. In writing, the truncated forms やば and やばっ function identically but signal different emphasis — やば is casual shortening, やばっ adds the sharp-cutoff emphasis of terminal っ (see A.2). The suffixed form やばすぎ (from 〜すぎる, "too much") intensifies further.

Examples

[context: SNS / reacting to a photo of a scenic view / positive] この景色《けしき》やばい This view is incredible

[context: LINE / reacting to food at a restaurant / positive] このラーメンやばすぎる This ramen is insanely good

[context: LINE / student after an exam / negative] 今日のテストやばかった。。。 Today's test was brutal...

[context: text message / seeing the time / alarmed] やばっ、もうこんな時間 Oh no, it's already this late

Dialogue

[written — LINE / two male university students / A sharing a find]

A: このチャンネルの動画やばい [The videos on this channel are insane]

B: どんなの？ [What kind?]

A: 料理《りょうり》系なんだけど、まじうまそう [Cooking stuff, but it looks seriously good]

B: 見てみるわ [I'll check it out]

A: 絶対《ぜったい》見て、やばいから笑 [Definitely watch, they're crazy good lol]

C. Particle and Structural Conventions in Written Casual

C.1 Particle drop in text

← 話し言葉の対応: 助詞《じょし》省略《しょうりゃく》 — particle omission in speech

← Spoken base: [2.1–2.4]

Formula: [N] + (は/が/を/に → ∅) + [predicate/question]

Register: ★★★ core Medium: written — LINE/text

Gap Note

Sections 2.1–2.4 cover particle omission in spoken Japanese, where intonation and shared context fill the gap left by the missing particle. In LINE messages, that prosodic support is absent — yet particles are dropped even more aggressively than in speech. Learners who have begun to accept particle drop in listening often re-expect particles in writing, because textbooks present writing as more complete than speech. LINE and text messages violate this expectation constantly. The result is that learners struggle to parse ultra-short messages like 明日暇？ or ご飯食べた？ where two or three particles are simultaneously absent.

How the transformation works

The mechanism is the same as spoken particle drop (see Section 2), but the threshold is lower in LINE messages. Brevity is the dominant pressure: messages are typed quickly on phones, and shared context between close contacts makes particles redundant. Topic は, subject が, object を, and even directional に are routinely omitted. The reader must reconstruct the particle from the relationship between noun and predicate.

Examples

[context: LINE / asking a friend about plans] 明日暇《ひま》？ (Are you) free tomorrow?

[context: LINE / asking about dinner] ご飯《はん》食《た》べた？ (Did you) eat?

[context: LINE / making plans] 駅《えき》何時？ What time (at the) station?

[context: LINE / casual check-in] 仕事《しごと》終《お》わった？ (Did you) finish work?

C.2 〜じゃん（文章）

← 話し言葉の対応: 〜じゃん / 〜じゃないか — spoken confirmation

← Spoken base: [6.4]

Formula: [S (plain)] + じゃん

Register: ★★★ core Medium: written — LINE/text, written — SNS

Gap Note

Entry 6.4 covers じゃん as a spoken confirmation particle with strong Tokyo register associations. What learners may not expect is how frequently じゃん appears in written casual Japanese — in LINE messages, tweets, and SNS comments. In speech, じゃん carries intonation that clarifies whether it is accusatory, friendly, or teasing. In writing, that intonation is absent, and the reader must infer tone from context alone. Learners who have just begun to recognize spoken じゃん may still fail to parse it in a LINE message, where it looks like a fragment of じゃない and triggers a negative-form misreading.

How the transformation works

Written じゃん functions identically to spoken じゃん — it asserts something the writer considers obvious or already established, seeking confirmation or pointing out what should be apparent. In text, it often appears with other written-casual markers (笑, っ, ー) that help signal tone.

Examples

[context: LINE / friend pointing out the obvious] それ昨日言ったじゃん I told you that yesterday, didn't I

[context: SNS reply / friendly teasing] めっちゃいいじゃん笑 That's really great, isn't it lol

[context: LINE / mild frustration] だから無理《むり》って言ったじゃん That's why I said it's impossible

Dialogue

[written — LINE / two friends / A bought something B recommended]

A: あのカフェ行ってきた [I went to that cafe]

B: どうだった？ [How was it?]

A: めっちゃよかった、ケーキやばい [It was really good, the cake was amazing]

B: でしょ？言ったじゃん笑 [Right? I told you lol]

C.3 〜なんだけど（SNS文末）

← 話し言葉の対応: 〜なんだけど（文末） — spoken trailing setup

← Spoken base: [7.5]

Formula: [S (plain)] + なんだけど (sentence-final, no continuation)

Register: ★★★ core Medium: written — LINE/text, written — SNS

Gap Note

Entry 7.5 covers the spoken sentence-final なんだけど as an implicit request or setup that leaves the ask unstated. In writing — particularly on SNS — this form takes on additional functions. A tweet ending in なんだけど is often a bid for sympathy, a complaint presented for an audience, or a humble-brag left for others to react to. Learners who have grasped the spoken form may still miss the performative dimension of written なんだけど, where the "audience" is not a single conversation partner but a public or semi-public readership. The unstated continuation is not "can you help?" but "isn't this interesting/terrible/relatable?"

How the transformation works

The mechanism is the same んだ explanatory frame + けど trailing conjunction as in speech (see 3.2 and 7.5). In writing, the absence of a follow-up clause is even more deliberate — the writer could have typed more, but chose not to. This converts the trailing form into a rhetorical device: it frames the preceding statement as something that deserves a reaction without explicitly asking for one.

Examples

[context: SNS post / complaining about weather / sympathy bid] 明日台風《たいふう》なのに出張《しゅっちょう》なんだけど So there's a typhoon tomorrow but I have a business trip...

[context: LINE / telling a friend about something exciting] 来月推《お》しのライブ当《あ》たったなんだけど I won the lottery for my favorite artist's concert next month...

[context: SNS / relatable complaint] また月曜日《げつようび》なんだけど It's Monday again...

[context: LINE / soft complaint to a friend] あの店《みせ》もう潰《つぶ》れてたんだけど That shop already closed down...

C.4 〜んだけど（LINE文末）

← 話し言葉の対応: 〜んだけど — spoken soft setup

← Spoken base: [3.2]

Formula: [S (plain)] + んだけど (sentence-final, in direct message)

Register: ★★★ core Medium: written — LINE/text

Gap Note

This form overlaps with C.3 but operates differently in context. Where C.3's なんだけど on SNS is performative (aimed at an audience), んだけど in LINE is directed at a specific person and functions as a soft setup — exactly as the spoken form does. The learner's comprehension gap is recognizing that a LINE message ending in んだけど is not a broken sentence or a transmission error. It is a complete communicative act: the writer is setting up a request, suggestion, or piece of news and leaving space for the reader to respond before the ask is made explicit. Textbooks that teach けど as "but" give learners no framework for reading this sentence-final use.

How the transformation works

The writer uses the んだ explanatory frame to mark the message as background information, then attaches けど to trail off — signaling that a follow-up exists but is being withheld. In LINE, this creates conversational space: the recipient is expected to reply (うん？ or どうしたの？), after which the writer delivers the actual request or point. It is a two-message structure where the first message is deliberately incomplete.

Examples

[context: LINE / asking a favor indirectly / to a close friend] 来週《らいしゅう》引《ひ》っ越《こ》しなんだけど So I'm moving next week...

[context: LINE / setting up a suggestion / couple] 今日ちょっと暇《ひま》なんだけど So I'm kind of free today...

[context: LINE / soft pre-complaint / coworker] 明日の会議《かいぎ》、資料《しりょう》まだなんだけど So the materials for tomorrow's meeting aren't ready yet...

Contrast with

C.3: 〜なんだけど（SNS文末） — performative/public vs. direct/private

D. Manga Speech Register Conventions

D.1 〜わよ / 〜ですわ

← 話し言葉の対応: [written-only form] — manga/anime convention; not standard modern speech

Formula: [S (plain/polite)] + わよ / ですわ

Register: ★ marked · feminine tendency · anime/fiction Medium: written — manga

Gap Note

Learners encounter わよ and ですわ in manga and anime and may attempt to use them as feminine speech markers, not realizing that these forms are almost entirely restricted to a specific fictional character type: the お嬢様《じょうさま》 (refined young lady, often wealthy or aristocratic). Real Japanese women do not speak this way in casual conversation. Genki mentions わ briefly as a feminine particle (see 4.9), but the leap from casual わ to the お嬢様 register of ですわ is never explained. Learners who hear ですわ and assume it is standard polite feminine speech will badly misread the character's social position and the scene's tone.

How the transformation works

ですわ combines the polite copula です with the feminine particle わ, creating a form that is simultaneously polite and marked as feminine — the hallmark of お嬢様 speech. わよ adds the assertive よ to わ, strengthening the claim while maintaining the feminine register. In manga, these forms are visual shorthand: the moment a character says ですわ, the reader knows they are encountering a refined, upper-class, or deliberately elegant female character.

Examples

[context: manga / お嬢様 character / introducing herself] わたくし、そんなことは存《ぞん》じませんわ I wouldn't know about such things.

[context: manga / wealthy female character / confident assertion] 当然《とうぜん》ですわよ Naturally.

[context: manga / お嬢様 / mild displeasure] それは困《こま》りますわ That would be troublesome.

D.2 〜じゃ / 〜じゃろう

← 話し言葉の対応: 〜だ / 〜だろう — plain copula and conjecture

Formula: だ → じゃ / だろう → じゃろう

Register: ★ marked · older/regional · anime/fiction Medium: written — manga

Gap Note

Learners see じゃ and じゃろう in manga and may confuse them with じゃない (negative) or the connective じゃ (as in じゃあ, "well then"). In manga, じゃ functioning as a copula and じゃろう as a conjecture marker are signals of a specific character type: the elderly male, often a mentor, village elder, or wise figure. These forms have roots in western Japanese dialects and classical grammar, but in modern manga they are pure character markers. No textbook explains this convention, so learners parse じゃ as negative or connective and misread the sentence entirely.

How the transformation works

じゃ replaces だ as the copula, and じゃろう replaces だろう as the conjecture form. Historically, these derive from western dialect forms (particularly 中国《ちゅうごく》 and 四国《しこく》 regional speech). In manga, they are detached from geography and attached to age and wisdom. A character who says そうじゃ where a normal speaker would say そうだ is being marked as old, authoritative, or both.

Examples

[context: manga / elderly mentor character / explaining something] それは違《ちが》うんじゃ That's not how it is.

[context: manga / old man / speculation] あいつならできるじゃろう That one can probably do it.

[context: manga / village elder / giving advice] 焦《あせ》ることはないんじゃ There's no need to rush.

D.3 〜だぜ / 〜だぜ！

← 話し言葉の対応: 〜だぜ exists in rough masculine speech but is amplified in manga

← Spoken base: [4.11]

Formula: [S (plain)] + だぜ / ぜ

Register: ★ marked · masculine tendency · anime/fiction Medium: written — manga

Gap Note

Entry 4.11 introduces ぜ as a spoken masculine assertion particle. In manga, だぜ and ぜ are dialed up significantly and attached to a specific character type: the ヤンキー (delinquent), the hot-blooded hero, or the rebellious tough guy. Learners who encounter 行くぜ！ in manga and only know ぜ from a brief textbook mention (if that) may not grasp that this particle is doing double duty — it is both asserting the statement and marking the speaker as a specific social type. The exclamation mark often accompanies だぜ in manga, reinforcing the aggressive energy.

How the transformation works

ぜ attaches to the plain form of a sentence as a masculine assertion particle (see 4.11). In manga, だぜ specifically — with the copula だ preceding ぜ — is the most common realization. The character type associated with だぜ is rough, confident, and often confrontational. The form is productive: any plain-form sentence can take だぜ to mark it as belonging to this register.

Examples

[context: manga / delinquent character / challenging someone] 俺《おれ》が一番《いちばん》強《つよ》いんだぜ I'm the strongest, y'know!

[context: manga / hot-blooded hero / rallying cry] 行くぜ！ Let's go!

[context: manga / tough male character / boasting] こんなの楽勝《らくしょう》だぜ This is a piece of cake!

D.4 〜のじゃ / 〜ぞよ

← 話し言葉の対応: [written-only form] — stylized classical; no modern spoken equivalent

Formula: [S (plain)] + のじゃ / ぞよ

Register: ★ marked · anime/fiction Medium: written — manga

Gap Note

Learners encounter のじゃ and ぞよ in fantasy manga, light novels, and historical drama settings and have no textbook anchor for either form. のじゃ combines the explanatory の (see 3.7) with the elderly-register copula じゃ (see D.2), creating a form that reads as both explanatory and archaic. ぞよ combines the masculine ぞ (see 4.10) with the classical particle よ in a construction that has no modern spoken use. Both forms are pure fiction conventions — signals that the story is set in a non-modern world or that the character has a deliberately antiquated manner of speaking. Learners who try to parse these with modern grammar rules will fail.

How the transformation works

のじゃ is structurally の (explanatory) + じゃ (archaic copula): "the thing is that..." delivered in an old-fashioned register. ぞよ fuses ぞ (strong assertion) with よ (informing), creating an archaic compound particle that signals authority and antiquity simultaneously. Neither form is productive in modern Japanese — they are fixed register markers for fantasy, period, and mythological character types such as dragons, ancient sages, gods, or royalty.

Examples

[context: fantasy manga / ancient dragon / explaining its power] 我《われ》が全《すべ》てを支配《しはい》するのじゃ I am the one who rules all.

[context: light novel / goddess character / issuing a decree] この者《もの》を許《ゆる》すぞよ I shall pardon this person.

[context: manga / old sage / explaining history] かつてこの地《ち》には偉大《いだい》な王《おう》がおったのじゃ Once upon a time, a great king lived in this land.

D.5 〜っす

← 話し言葉の対応: 〜です — polite copula, compressed

← Spoken base: [5.13]

Formula: です → っす / す

Register: ★★ common · masculine tendency · youth Medium: written — manga, written — LINE/text

Gap Note

Entry 5.13 covers っす as a spoken casual-polite hybrid, common among male university students and in service-industry speech. In manga, っす is the defining speech marker of the 後輩《こうはい》 (junior) character type — the younger male who wants to show respect to a senior but not in a stiff, formal way. Learners may recognize です but not connect っす to it, especially in manga where it appears without the surrounding phonetic context of speech. The comprehension failure is twofold: the learner does not decode っす as です, and even if they do, they miss that it signals a specific social position (junior to senior, casual but deferential).

How the transformation works

です compresses to っす through the same phonological process as in speech: the /de/ syllable reduces, leaving the glottal stop っ and the /su/. In manga, this compression is rendered consistently in the speech bubbles of 後輩 characters, sports team juniors, and workplace newcomers. The form is productive — any sentence that could end in です can end in っす: いいっす, 大丈夫《だいじょうぶ》っす, そうっす.

Examples

[context: manga / junior club member to senior / agreeing] 了解《りょうかい》っす Got it! (casual-polite)

[context: manga / kouhai character / reporting] 先輩《せんぱい》、終わったっす Senpai, I finished.

[context: manga / young male employee / casual deference] まじっすか Seriously? (casual-polite)

[context: LINE / male junior to a senior friend / agreeing to plans] いいっすよ、行きましょう Sounds good, let's go.

Dialogue

[written — manga / sports team setting / junior A talking to senior B after practice]

A: 先輩、今日の練習《れんしゅう》やばかったっす [Senpai, today's practice was brutal]

B: おー、お疲れ [Oh, good work]

A: 明日も朝からっすよね？ [Tomorrow's from the morning too, right?]

B: そうだぜ、気合《きあい》入《い》れろよ [That's right, get fired up]

A: はい、頑張《がんば》るっす！ [Yes, I'll do my best!]

道 — Colloquial Japanese Patterns