Green’s Dictionary of Slang

1. to seduce.

1909		Goodwin’s Wkly (Salt Lake City, UT) 25 Dec. 5/2: Some of those jealous cats [...] used to sing to me, ‘Why Don’t You Get a Husband of Your Own?’ and I just thought I’d show them by copin’ off Jim. Aint I the cute kid?
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 185: Wishing he’d [...] copped off a broad downtown.
1943		J. Phelan Letters from the Big House 80: He’s coppin’ off Minnie all the time as well!
1999		Guardian G2 27 Oct. 11: One woman copped off with a fireman 15 years younger than her.
2000		M. Manning Get Your Cock Out 134: Gerry hand’t seen his guitarist so happy since he copped off with some junkie supermodel chick in New York.

2. (US) to embark on a (non-sexual) relationship with, to befriend.

1914

H. Hershfield Abie the Agent 14 Dec. [synd. cartoon strip] As long as Skinny’s old man won’t let him talk to Van no more I guess I’ll cop him off myself [...] I’ll go up to his shack and call on him.

3. to embrace sexually, to indulge in petting; usu. cop off with.

1977		A. Bleasdale Who’s Been Sleeping in my Bed 34: He was tryin’ t’cop off with her an’ she wasn’t havin’ any.
1986		Eble Campus Sl. Nov. 3: cop off – go off with a member of the opposite sex for various kinds of sex.
1998		A. Warner Sopranos 318: I seriously copped off with the bouncer, shagged him and I really regret it.
1999		Guardian 24 Feb. 3: Stuart [...] cops off with Nathan, 15, and still a virgin (not for long though).
2001		N. Barlay Hooky Gear 180: Middle-age beard in a jacket tryin to cop off with a mix-race bint half his age.