From Longman Dictionary of Contemporary Englishhair triggerˌhair ˈtrigger noun [countable] PMWa trigger1 on a gun that needs very little pressure to fire the gun
Examples from the Corpus
hair trigger• Getting rid of weapons reduces the risk of accidental, unauthorised, hair trigger or pre-emptive use.hair-triggerˈhair-trigger adjective [only before noun] easily made angry his hair-trigger temperamentExamples from the Corpus
hair-trigger• Mix in the hair-trigger rage that can result from steroid use and, well, stand back, ladies and gentlemen. 