Green’s Dictionary of Slang

(20C+ use mainly US) to act as a confederate in a gambling game.

1811		Lex. Balatronicum n.p.: To Cap. To support another’s assertion or tale. To assist a man in cheating. The file kidded the joskin with sham books, and his pall capped; the deep one cheated the countryman with false cards, and his confederate assisted in the fraud.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1859		Matsell Vocabulum 17: cap. To join in, ‘I will cap in with him’ — I will appear to be his friend.
1873		J. O’Connor Wanderings of a Vagabond 216: Every aristocratic skinning-house has its aristocratic loafers to ‘cap’ its games, who, from time to time, receive the acknowledgments of the proprietors, in the shape of a five or ten dollar bill.
1887		G. Devol Forty Years a Gambler 74: He [...] told me he was the brother of Mike Carroll, and he wanted to cap for me.
1914		S. Lewis Our Mr Wrenn (1936) 50: Now don’t try to do me out of my bit or I’ll cap for some other joint.
1933		Ersine Und. and Prison Sl. 24: cap. v. To shill; to act as a come-on man.
1938		H. Asbury Sucker’s Progress 205: River gamblers seldom operated alone [...] They capped and roped for one another’s banking games.
1950		Goldin et al. DAUL 40/2: Cap. To serve as a fast-talking aide to a confidence man, driving home with emphasis and timely remarks the come-on scheme.