Green’s Dictionary of Slang

(orig. US) of a group of men, to have sexual intercourse (voluntary or otherwise) with a single woman.

1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 195: The time we gang-shagged that little bitch Iris.
1938		‘Justinian’ Americana Sexualis 24: Gang-Shag, v. See Gang-Fuck.
1968		G. Legman Rationale of the Dirty Joke (1972) I 267: That there is little or no vaginal lubrication in the drugged girl [...] is solved in part by gang-rape or ‘gang-shagging’.
1975		F. Russell A City In Terror 137: One woman going along Atlantic Avenue [...] was seized by a group of young men, knocked to the ground, and gang-shagged.
1988		J. McDonald Dict. of Obscenity etc.
2004		R.W. Doty Man and Woman, War and Peace 329: Was certainly a wild ship that nite with all the ship’s officers trying to gang shag these 4 gals and I guess they succeeded quite well too.