Irresponsible in the use of firearms; eager to fire a gun
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with trigger happy
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