Green’s Dictionary of Slang

cap n.3

[capper n.1 (1)]

(US Und.) the act of ensnaring a victim into a confidence game.

[US]G. Devol Forty Years a Gambler 71: The barkeeper handed me the $500 he had won. I gave him $200 for his cap.
[US]B. Jackson Get Your Ass in the Water (1974) 92: You went on back upstairs and called all the cats around you, / told ’em what a cap you had made on the snap, but you didn’t know the bitch was leavin’ town.