Trigger-Happy Meaning

(idiomatic) Inclined to behave recklessly or violently at the slightest provocation.

Example:   The bystanders were indiscriminately gunned down by trigger-happy gang members.
  Be careful when you work near him; he's trigger-happy with that chain-saw.
  He was so trigger-happy that he wheeled and punched me when I brushed by him in the hall.