Green’s Dictionary of Slang

cap n.5

[cap v.5 ]
(orig. US black)

1. a rejoinder.

[US] ‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.

2. an insult aimed at someone’s family; used as part of a ritualized interchange.

[US]M. Braly On the Yard (2002) 32: An enormous guard [...] stepped through a metal door in time to catch Red’s last cap.
[US]F. Salas Tattoo the Wicked Cross (1981) 129: The crowd shouted at the good caps, laughed at the funny ones, and jeered and moaned at the poor ones.
[US]Too $hort ‘Cusswords’ [lyrics] I got cap for cap, you never heard / So fresh again with cusswords.