Green’s Dictionary of Slang

1. a large pistol or revolver.

1861		‘Camp Phrases’ in Chicago Trib. 11 Nov. 2: A tent is jocularly termed ‘the canvas,’ a sword is a ‘toad sticker,’ and any of the altered patterns of muskets are known as ‘howitzers.’.
1874		J.G. McCoy Sketches of the Cattle Trade 205: An affront or slight, real or imaginary, is cause sufficient for him to unlimber one of [sic] more ‘mountain howitzers’ invariably found strapped to his person.
1903		‘O. Henry’ ‘The Atavism of John Tom Little Bear’ in Rolling Stones (1913) 38: Here, you pappoose [...] what are you gunning for with that howitzer?
2020		S.A. Crosby Blacktop Wasteland 114: She had a hand held howitzer pointed at Quan.

2. in pl., large female breasts.

1970		‘Troy Conway’ Cunning Linguist (1973) 89: Her twin howitzers danced in my eye.
2000		G. Pelecanos Shame the Devil 47: Grace, the waitress with the howitzers, had brought the beer in to him after lunch.

3. the penis.

1972		R.A. Wilson Playboy’s Book of Forbidden Words.
1975		J. Roe The Same Old Grind 143: He followed her with his high-angle howitzer. ‘Take it, cocksucker!’.