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1. Eager to use a weapon, i.e. to pull the trigger (of a gun). I know you're trigger-happy, but quit shooting or you'll scare off all the animals!
2. Having a tendency to do something or engage in or begin some activity with even the smallest level of enticement or provocation. Whenever I'm editing, I always get a little trigger-happy with the delete key and end up having to add some things back later. Don't pass to Rick. He's trigger-happy and shoots ever time he gets the ball.
Inclined to act violently at the slightest provocation, as in They feared that the President was trigger happy and would send in troops at the drop of a hat . This expression alludes to being too eager to fire a gun. [c. 1940]
If someone is trigger happy, they are too willing to use a gun. The firing continued throughout the night, trigger happy soldiers making sleep impossible.