Green’s Dictionary of Slang

(US) to fire a bullet.

1866		C.H. Smith Bill Arp 118: He rode up to the militia and pulling out his repeater, exclaimed, with uncommon gravity, ‘Lay down, meelish, I am going to bust this cap’.
1877		New Orleans Daily Democrat (LA) 25 July 1/3: Somebody asks ‘where are the Bucktail Rangers?’ and echo answers ‘Lie down melish, I’m gwine to bust a cap’.
1886		Staunton Spectator (Staunton, VA) 12 May 1/5: But when the cannons boom you hear the cry, ‘Lie down, militia; I’m going to bust a cap’.
1897		Nat. Trib. (Wash., DC) 21 Dec. 5/4: ‘I’d like awfully well t’ bust a cap at that brash young Lieutenant, afore he gets clean away’.
1909		Pensacola News Jrnl (Pensacola, FL) 6 Jan. 4/4: Leo Turner, a mite of a negro boy, stood up for Julius Underwood, another pickaninny, to show him how to ‘bust a cap’ on a small ‘unloaded rifle’.
1911		Galveston Dly News (Galveston, TX) 11 May 6/5: It must be rather binding on the American soldiers at El Paso to have to stand in ranks and watch the battles on the other side of the river without being allowed to ‘bust a cap’.
1921		Palmyra Spectator (Palmyra, MO) 2 Feb. 9/2: If I was goin’ up to Dixon to shoot Chrischuns I’d never bust a cap on that kind of a man.
1934		Denton Record-Chron. (TX) 24 May 5/7: Frank Hamer, strapping former Texas Ranger, leader of the ambush—’I hated to bust a cap on a woman, especially when she was sitting down’.
1941		Tallahassee Democrat (Tallahassee, FL) 20 Jan. 3/1: ‘’Sno use, Pickles, the boys sure must bust a cap and burn powder and if the game is sorter shy then they soothe their restless souls at the traps’.
1952		Arizona Republican (Phoenix, AZ) 23 Dec. 38/2: I was up to Roosevelt over the weekend for some goose hunting [...] but we never got to bust a cap.
1964		R. Helmer Stag Party 85: Why bust a cap when you can do it a cheaper way?
1972		O. Hawkins Ghetto Sketches 12: Haul ass ’less you want t’ get a cap busted in it!
1988		(con. 1930s) C.E. Lincoln The Avenue, Clayton City (1996) 151: Vernon steady bustin’ caps up they ass with ol’ Hargis’s thirty-nought-thirty.
1988		D. Woodrell Muscle for the Wing 202: Don’t bust that cap on me, man.
1993		Dr Dre ‘187’ 🎵 Walkin’ down the streets of L.A. / Stay strapped cause niggas bust caps every day, yo.
2000		Ebonics Primer at www.dolemite.com 🌐 bust a cap Definition: to fire a weapon. Example: Fuck wit me and I’ll bust a cap in yo ass.
2000		G. Pelecanos Shame the Devil 76: [He] raps about busting a cap in someone’s head, fucking women in the ass, all that.
2008		Simon & Burns ‘More with Less’ Wire ser. 5 ep. 1 [TV script] I shoulda busted a cap in Pookie’s ass my own self.

to fire a weapon; to shoot someone.

1865		in B.L. Ridley Battles and Sketches of the Army of Tennessee (1906) 454: My men halloo out: ‘Lie down, Mr. Govan, Mr. Bate is now gwine to pop a cap’.
1927		Lincoln Jrnl Star (NE) 6 June 5/3: A colonel cannot let his regiment retreat whenever the enemy pops a cap.
1937		El Paso Herald-Post (El Paso, TX) 10 Aug. 5/1: He and his lieutenant dragged a Marine gun up to the front and then didn’t pop a cap.
a.1953		‘Death Row’ in D. Wepman et al. Life (1976) 118: I popped four caps through his chest with my piece.
1971		T. Thackrey Thief 408: So, fuck him – I popped a cap.
1981		J. Wambaugh Glitter Dome (1982) 128: Schultz and Simon could never prove that on some occasion Samuel Billings hadn’t tried to pop a couple of caps at a cat or a rat.
1993		Tarantino & Avery Pulp Fiction [film script] 158: A cold-blooded thing to say to a motherfucker ’fore you popped a cap in his ass.
1996		Bynum & Thompson Juvenile Delinquency 290: The man popped a cap at cuzz while he laid a drag on a freak sitting on the benzo.
1997–2001		Online Sl. Dict. 🌐 pop a cap v 1. to shoot. 2. to kill with a firearm. (‘I’m gonna pop a cap into that bitch when I see her next!’).
2000		F.X. Toole Rope Burns 165: If some fool of whatever color was coming to kill him, he’d pop a cap in his ass in a heartbeat.
2004		P. Howard PS, I Scored the Bridesmaids 149: I’m gonna pop a cap in the mother’s ass.
2013		T. Black ‘I Want Candy’ in Killing Time in Las Vegas [ebook] That bastard tries to bigfoot me one more time, I’m popping a cap in his wide old ass.
2020		S.A. Crosby Blacktop Wasteland 169: ‘Get out the car, Bug. No need to give the cops a reason to pop a cap in you’.

(US) to fire a shot.

1864		in S.C. Wilson Column South (1960) 218: He said, ‘To hell with the Yankees,’ and snapped a cap [HDAS].
1893		R.G. Hampton Major in Wash. City 155: He’s as game a man as ever snapt a cap.
1929		D. Hammett Red Harvest (1965) 70: I yanked the gun out and snapped a cap at Thaler.