trigger finger

noun

any finger, usually the forefinger, that presses the trigger of a gun.
either forefinger, depending on which hand is dominant.
Pathology. difficulty in straightening or bending a ring finger, caused by inflammation and thickening of its tendon.

Origin of trigger finger

First recorded in 1820–30

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

But even as it did so Peter Champneys's trigger-finger moved.
The Purple Heights
Marie Conway Oemler
Gripping his revolver hard, his trigger-finger steady, the corporal fired again, and another of his foes fell.
The adventures of Kimble Bent
James Cowan
The speaker smoothed the barrel of his rifle with mittened hand, which had, however, a trigger-finger free.
Northern Lights
Gilbert Parker
Suppose that the Carolina trigger-finger slipped, as Hunter had nonchalantly admitted might happen: what then?
Slippy McGee, Sometimes Known as the Butterfly Man
Marie Conway Oemler
The brain, head, and eyes and trigger-finger must all work in harmony or you will never be a good shot.
Outdoor Sports and Games
Claude H. Miller