1No good; useless; broken — describes something that no longer serves its purpose or can't be used. Common in phrases like もうだめだ ('it's done for') or これじゃだめだ ('this won't do'). Usually written in kana rather than 駄目.
このプリンターは古くてもうだめだ。買い替えるしかない。
This printer is old and finished — we have no choice but to replace it.
傘が風で骨が折れて、もうだめになった。
The wind broke the spokes of my umbrella, and it's done for.
そんな曖昧な説明じゃだめですよ。もっと具体的に話してください。
That kind of vague explanation won't do — please be more specific.
2Hopeless; wasted; in vain — used when an effort or attempt fails or comes to nothing, typically in だめになる ('end up ruined / spoiled') or だめだった ('it didn't work out').
雨で楽しみにしていたピクニックがだめになった。
The picnic I'd been looking forward to was ruined by the rain.
面接を受けに行ったけど、緊張しすぎて結果はだめだった。
I went to the interview, but I was too nervous and it didn't go well.
せっかくの旅行計画も、台風が来てだめになってしまった。
Our long-awaited trip plan was wrecked when a typhoon hit.
3Cannot; must not; not allowed — expresses prohibition or refusal. Common in 〜してはだめ / 〜したらだめ patterns, used in everyday warnings and especially to children.
ここでタバコを吸ってはだめですよ。
You can't smoke here.
宿題が終わるまでゲームをしてはだめだよ。
You're not allowed to play games until your homework is done.
嘘をついちゃだめだよ、ちゃんと本当のことを言って。
You mustn't lie — tell me the truth.
そんな冷たい言い方をしたらだめだよ、相手が傷つく。
You shouldn't say things that coldly — it hurts the other person.
4(Go term) A 'neutral point' — an intersection on the board that, at the end of a game, belongs to neither player. A specialist term used only by Go players, not heard in general conversation.
終盤になって、お互いに駄目を詰める作業に入った。
As the endgame began, both players started filling in the neutral points (dame).
5An interjection — だめ！/ ダメ！ on its own, used to strongly stop or refuse the listener's action. In manga, social media, and casual speech, the katakana form ダメ is especially common.
ダメ！そのボタンは絶対に押さないで！
No! Don't press that button — absolutely not!
ダメ、ダメ、もう一杯はやめておきなさい。
No, no — don't have another drink.
「触ってもいい?」「だめ!」
"Can I touch it?" "No!"