Green’s Dictionary of Slang

1. (gambling) for a bystander to persuade potential victims to avoid a (poss. crooked) gambling game.

1887

G. Devol Forty Years a Gambler 91: I was surprised to see you back-capping my game, for I could see you were a sporting man.

2. to speak evil of someone, so as to spoil their game.

1879		Cincinnati Enquirer 7 Sept. 10/7: Back-cap, Blast – Is to speak ill of a person or play, the former being the term most generally used, and we regret to say with much cause, for among no other class of people [i.e. actors] does the tendency to back-cap exist.
1883		Nat. Police Gaz. (NY) 17 Nov. 10/2: There are a lot of envious fellows who are jealous of John’s prowess [...] ands they try to back-cap him whenever they can.
1896		Ade Artie (1963) 57: She’s a nice girl, though. I don’t want to back-cap her.
1902		Ade Forty Modern Fables 40: When he applied for a Consulate, all the influential Moguls of the Party signed his Petition. Then they sat down and wrote Private Letters to Back-Cap him.

3. to insult someone, esp. by disparaging their family.

1889		Century Dict. I 413/1: Backcap [...] To depreciate or disparage (U.S. slang).
1902		Ade Girl Proposition 52: It was a shame to back-cap her own Tribe, just when they were pulling for her, but she had to do it.
1928		Hearst’s International Aug. 34/2: Fine specimen you are — back-cappin’ your own neighbors to town trash! [DA].
1937		Duckett & Staple ‘Double Feature’ in N.Y. Age 10 July 7/2: Joe Louis very neatly back-capped a lot of people.

4. (US black) to answer, e.g. a letter; to respond to an advance.

1942		D. Burley N.Y. Amsterdam Star-News 31 Jan. 16: The one about flying his banter a kite from out of his vest which the banter didn’t cop so he couldn’t back-cap.
1945		D. Burley N.Y. Amsterdam News 3 Mar. 14A: I’ve been for you a long, long time an’ you don’t backcap.