Green’s Dictionary of Slang

(orig. US) overemotional, keen to put action before thought.

1947		(con. 1944) J.H. Burns Gallery (1948) 333: You’re goddam triggerhappy for a medic.
1951		T. Driberg Best of Both Worlds (diary) 24 Apr. (1953) 19: Don’t trust anybody, but don’t get trigger-happy.
1956		‘Ed Lacy’ Men from the Boys (1967) 39: Sort of a throwback to the old-style trigger-happy hood.
1961		R. Gover One Hundred Dollar Misunderstanding 36: You ain never gonna go so long wiffout pussy you gits that trigger-happy agen.
1991		C. Hiaasen Native Tongue 242: It brought back the memory of Monkey Mountain and the trigger-happy baboon.
2023		Rayman & Blau Riker’s 186: [S]omebody got trigger happy and did the fucking thing before the lock-in.