hair trigger

noun
a trigger that allows the firing mechanism of a firearm to be operated by very slight pressure.

Origin:
1815–25

Dictionary.com Unabridged

hair-trigger

[hair-trig-er]
adjective
easily activated or set off; reacting immediately to the slightest provocation or cause: a hair-trigger temper.

Origin:
1885–90

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
Cite This Source Link To hair trigger
Collins
World English Dictionary
hair trigger
 
n
1.  a trigger of a firearm that responds to very slight pressure
2.  informal
 a.  any mechanism, reaction, etc, set in operation by slight provocation
 b.  (as modifier): a hair-trigger temper

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Example sentences
He's always been a guy without boundaries, a chronic inappropriate kidder with
  a hair trigger temper.
But for less ham-handed duelists, the hair trigger allowed not only a faster
  shot but a more accurate one, too.
Volatility goes up, because firms can and do enter and exit positions on a hair
  trigger.
Without this hair trigger electrical system, life would be more than calm.
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