Green’s Dictionary of Slang

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

1887		G. Devol Forty Years a Gambler 71: The barkeeper handed me the $500 he had won. I gave him $200 for his cap.
1964		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.

2. (UK black) exaggeration, nonsense; a lie, thus phr. of denial when queried no cap, that’s the truth!

2018-20		T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 Cap - lie.
2018-20		T. Thorne (ed.) ‘Drill Slang Glossary’ at Forensic Linguistic Databank 🌐 No cap/cappin’ - no word of a lie, ‘I’m telling the truth’.
2022		Digga D. ‘Noughty by Nature’ [intro]🎵 The shit you hear them rappin’ about is all cap, everything's exaggerated.
2023		Sydney Morn. Herald 18 Nov. 🌐 Cap/No Cap: Cap means to lie, while no cap is telling the truth.