Green’s Dictionary of Slang

1. to shoot, to fire a gun; thus capping, gunfire.

1981		M. Baker Nam (1982) 54: All of a sudden, we heard capping by us – Cap-cap, Cap-cap.
1997		E. Little Another Day in Paradise 212: Ain’t seen Payaso since he capped that mayate from Sho-line.
2002		Teen Lingo: The Source for Youth Ministry 🌐 cap v. [...] 2. n. To shoot someone.

2. to kill, to murder, to shoot dead.

1972		C. Fuller, Jr ‘Love Song for Wing’ in King Black Short Story Anthol. (1972) 147: Tomorra we gonna’ CAP you, Blood! You!
1985		Duke Bootee ‘Broadway’ 🎵 Tourist with a camera, ’bout to get capped.
1997		G. Sikes 8 Ball Chicks (1998) 170: If you see her, tell her we’re gonna cap her.
2002		J. Ellroy ‘Little Sleazer & the Mail-Sex Mama’ in Destination: Morgue! (2004) 167: He capped a cat named Dag Drollet and did joint time.
2008		T. Piccirilli Fever Kill 16: The homicides he’d witnessed, the bad guys he’d capped.
2017		D. Winslow The Force [ebook] They’ll cap you and fuck you in the entry wounds.