This means you must not do something. It is a casual way to say "you are not allowed to."
Verb [ちゃ-form] + だめPrimary
Verb [じゃ-form] + だめ
This means you must not do something. It is a casual way to say "you are not allowed to."
「もう夜遅いから、テレビを見ちゃだめだよ。」
It's already late at night, so you're not allowed to watch TV.
「危ないから、ここで遊んじゃだめ！」
It's dangerous, so don't play here!
子供の頃、夜９時以降は起きていちゃだめだった。
When I was a child, I wasn't allowed to be up after 9 PM.
人の悪口を言っちゃだめだって、お母さんにいつも言われてるでしょ？
You're always being told by your mom that you shouldn't speak ill of others, right?
ここでタバコを吸っちゃだめじゃないですか。常識でしょう。
You're not supposed to smoke here, are you? It's common sense.
This phrase can sound like you are talking down to someone. It is like a parent telling off a child. Close friends can use it. But it can sound rude if you don't know someone well. You can also use it when talking to yourself. For example, "I must not forget!"
This is different from "〜てはいけません" (te wa ikemasen). That phrase is more formal. You use it in writing or with people you do not know well. "〜ちゃだめ" is for casual talk. You use it with friends or family. A parent might use it with a child.
Don't use this when you need to be polite. For example, don't say it to your boss. The form changes based on how the verb's "-te" form ends. If it ends in "-te", you use "-cha dame". If it ends in "-de", you use "-ja dame". "Suru" becomes "shicha dame". "Kuru" becomes "kicha dame".
Contrasts with(3)
Less formal alternatives(1)
Dialect / slang variants(1)
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