hair-trigger

adjective

1. easily activated or set off; reacting immediately to the slightest provocation or cause: a hair-trigger temper.

noun

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

noun

1. a trigger of a firearm that responds to very slight pressure
2. informal
   1. any mechanism, reaction, etc, set in operation by slight provocation
   2. (as modifier)a hair-trigger temper

n.

1830, originally a secondary trigger in a firearm which sprung free a mechanism (hair) which, when set, allowed the main trigger (n.) to be released by very slight force. Figurative use by 1841.