One way to hold this word: something dangerous is already in place, and nothing is happening yet. That is the whole picture. A clock is running inside it, the hour has been chosen, and until that hour arrives the room looks perfectly ordinary. What makes the word bite is not the damage, which you never see, but the waiting — the gap between how calm it looks now and what is certain to arrive. Every use of the word keeps that gap. Only the thing doing the waiting changes.
可以想成這樣一個畫面：危險的東西已經放好了，而現在什麼事都還沒發生。整個字就是這個畫面。裡面有個鐘在走，時間也早就設定好了，在那一刻到來之前，房間看起來再平常不過。這個字真正咬人的地方不是爆炸——爆炸你根本看不到——而是那段等待：眼前這麼平靜，跟等一下一定會來的事之間，那道落差。這個字的每一種用法都保留了那道落差，換掉的只是「等在那裡的東西」是什麼。
From the word’s history
Wiktionary records the word as from time + bomb: a bomb tied to a clock rather than to the moment.
維基詞典把這個字記成 time 加 bomb：一顆綁在鐘上、而不是綁在當下的炸彈。名字本身就已經把「等」寫進去了。
Wiktionary, "time bomb"
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