hair trigger

Short definition from Wiktionary

Noun

1. 1
   Alternative form of hair-trigger.
2. 2
   Any unstable situation.
3. 3
   A trigger that will set off a firearm when even a very small pressure is applied.

Adjective

1. 1
   Alternative form of hair-trigger.
2. 2
   Unstable; in a situation that could be worsened by the slightest mistake.
3. 3
   Easily angered; having a short temper.