None of this is in a textbook, and all of it is in every conversation, drama and YouTube video. These are casual words: they work with friends and people your own age, and they are wrong at work, with strangers, and with anyone older than you.
Reactions you will hear constantly
Japanese conversation runs on reactions. These are the ones that do most of the work.
やばい
yabai
Insane / awful / amazing.casual
Originally "dangerous", now means whatever the tone says. やばい can be a disaster or the best meal of your life.
まじ？
maji?
Seriously?casual
まじで as an intensifier: まじでうまい, "seriously good".
うそ！
uso!
No way! (lit. "lie")casual
Surprise, not an accusation. Nobody is calling you a liar.
すごい / すげー
sugoi / sugee
Amazing, impressive.casual
すげー is the flattened male-leaning version. Same for やばい to やべー.
なるほど
naruhodo
Ah, I see. That makes sense.
Safe in almost any register, unlike the rest of this list.
えっ
e
Huh? / Wait, what?casual
A whole reaction in one syllable. Length and pitch carry the meaning.
Agreeing and going along
だよね
da yo ne
Right? / Exactly.casual
The casual cousin of ですね. Agreeing and inviting agreement at once.
そうそう
sou sou
Yes, that, exactly.casual
たしかに
tashika ni
True, fair point.
Conceding something you had not thought of. Extremely common.
わかる
wakaru
I feel that. / I know, right.casual
Not "I understand the information", more "I have felt exactly that".
りょうかい / りょ
ryoukai / ryo
Got it. / K.casual
りょうかい is military in origin and now just casual. りょ is the texting abbreviation.
Describing things
かわいい
kawaii
Cute.
Far broader than English "cute": food, stationery, a grown adult, a shop front.
めんどくさい
mendokusai
What a hassle. Cannot be bothered.casual
One of the most used words in the language. Often shortened to めんどい.
めっちゃ
meccha
Really, super.casual
Kansai in origin, now used everywhere. めっちゃおいしい.
超〜
chou ~
Super, mega.casual
Stick it on the front of anything: 超うまい, 超いい.
ださい
dasai
Uncool, tacky, dated.casual
びみょう
bimyou
Eh, so-so, not great.casual
A polite way to say no. If someone calls your idea びみょう, they dislike it.
Talking about people
イケメン
ikemen
Good-looking guy.casual
オタク
otaku
Obsessive fan, geek.casual
Mildly negative from a stranger, worn with pride about yourself. 〜オタク works for any hobby.
天然
tennen
Adorably clueless, away with the fairies.casual
Affectionate, not an insult. Literally "natural".
キャラ
kyara
Someone’s persona or type.casual
From "character". キャラが濃い means someone is a big personality.
ドタキャン
dotakyan
A last-minute cancellation.casual
From 土壇場 (last moment) plus "cancel". Used as a verb: ドタキャンされた.
Texting and internet
w / 笑
w / wara
lolcasual
From 笑い. More w means more laughing: wwww. 草 (grass) came from a row of w looking like grass, so 草 also means lol.
ググる
guguru
To google.casual
A foreign noun turned into a Japanese verb. Conjugates normally: ググった.
インスタ映え
insuta-bae
Instagrammable.casual
とりま
torima
Anyway, for now.casual
Squashed from とりあえず、まあ. Texting and speech both.
おつ / お疲れ
otsu / otsukare
Good work. / Later.casual
Short for お疲れさま. Doubles as goodbye among colleagues and friends.
Handle with care
These are real and common, and they will land badly if you misjudge the room.
うざい
uzai
Annoying.casual
Genuinely rude about a person. Fine about a situation, risky about someone present.
きもい
kimoi
Gross, creepy.casual
From 気持ち悪い. Harsh. Aimed at a person it is an insult, not banter.
おまえ
omae
You. (blunt)casual
Fine between close friends, aggressive otherwise. As a learner, use the person’s name instead.
やば
yaba
Whoa. (clipped やばい)casual
Muttering it to yourself is safe. Aiming it at someone’s work is not.
Worth knowing
Register is the whole game
Slang with a shop assistant, a teacher or a colleague reads as rude rather than friendly. Default to polite Japanese and switch only once the other person does.
Understand it before you use it
The value of this sheet is comprehension. Recognising やばい and めんどくさい in the wild is worth more than deploying them yourself.
It dates fast
Reaction words like やばい and まじ have been around for decades. Internet coinages turn over every few years, so treat the texting section as a snapshot.
One-line rule: Learn these to understand, not to perform. Slang signals closeness, so using it before the relationship is there is the one mistake that actually causes offence.