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.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
To Cap-1 * Cap AcquaintanceNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes