Green’s Dictionary of Slang

(US) a gunman, a gangster.

1938		B. Conlon ‘Rope Meat’ in Wild West Weekly 22 Oct. 🌐 Pop’s elderly wife had [...] run after the gunny, escaping a wild bullet.
1946		C.S. Montanye ‘Don’t Meddle with Murder’ in Thrilling Detective May 🌐 A gunny named Stangl has been after your friend.
1957		D.B. Sands ‘Some Colloquialisms of the Handgunner’ in AS XXXII:3 193: gunny, n. A gun enthusiast.
1968		J. Alard He Who Shoots Last 216: ‘Don’t gimme da shits. You ain’t no gunnie’.
1970		‘Troy Conway’ Cunning Linguist (1973) 75: The poor gunnie-goon had a one-two-three kind of brain.
1988		T. Harris Silence of the Lambs (1991) 311: The chief gunny, Brigham.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 206: Oh wrap me in my guns and ammo, / And bury me down deep below, / Where Sid and the Buggster can’t get me, / Down where all gunnies go.
2004		(con. 1960s-70s) T. Taylor Top Fellas 21/1: A ‘Gunny’ [...] was a bloke whose pocket bulged with a hidden persuader, a gun-man.
2017		S.M. Jones August Snow [ebook] ‘Maybe something to do with that sizable callous on your right inner palm, eh, gunny?’.