Quick to respond or react; touchy; sensitive (1940s+)
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 quick on the draw
quick on the trigger. Rapid in acting or reacting, as in You have to be quick on the draw if you want to find low-rent housing here, or Bruce was quick on the trigger when it came to answering questions. The first expression came from the American West's gunslingers and was broadened to mean “a quick reaction” in the first half of the 1900s. The variant originated about 1800.