hair-trigger

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?

Unstable; in a situation that could be worsened by the slightest mistake.

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.