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If someone is hair-trigger, they get angry very easily — a small thing is enough. We often say a hair-trigger temper: the anger is already loaded, and almost anything can set it off. The picture comes from a gun. A hair-trigger is a trigger so light that even a touch as small as a hair could fire it. From there the word jumped to people. Informal to everyday. Though actually you will also hear it about situations that can explode from a tiny push; the sense you need here is the person who flares up fast.
一觸即發、火爆（常說 hair-trigger temper＝火爆脾氣、一點就爆）
組成hair + trigger. A trigger is the part you pull to fire a gun; a hair-trigger needs almost no pull.
同家族的字
- triggerthe gun part, and also the thing that sets a feeling off; here the trigger is far too easy to set off
原始詞典資料3 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
hair-triggernoun/ˈhɛɹˌtɹɪ.ɡɚ/
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A trigger that will set off a firearm when even a very small pressure is applied.
- […]the hunter brings the sight of his rifle to bear; lightly and delicately his finger presses upon the hair-trigger.
- Is your laugh hung on a hair-trigger?—that is, is it critical, or can you get it off easy?
hair-triggeradj/ˈhɛɹˌtɹɪ.ɡɚ/
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Unstable; in a situation that could be worsened by the slightest mistake.
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Easily angered; having a short temper.
- "Gus," the latter began, "am I right in assuming that you possess a reasonable amount of influence with that hair-trigger partner of yours, Live Wire Luiz?"
- [I]n the 575 days since [Oscar] Pistorius shot dead his girlfriend Reeva Steenkamp, there has been an unseemly scramble to construct revisionist histories, to identify evidence beneath that placid exterior of a pugnacious, hair-trigger personality.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。