Green’s Dictionary of Slang

(orig. US, also gang shack, gang-shay) sexual intercourse between a single woman and a group of men (either voluntary or as mass rape); also in gay use.

1927		F.M. Thrasher Gang 238: The gang shag is an institution peculiar to gangs and clubs of this neighbourhood. [Ibid.] 267: Gang shag—sex party in the alley.
1931–4		D. Clemmer Prison Community (1940) 332/1: gang shag, n. Sexual intercourse between a woman and a group or gang of young men.
1932		(con. 1910s) J.T. Farrell Young Lonigan in Studs Lonigan (1936) 144: Davey said most of the guys were having a gang-shag at Iris’. ‘Yeah!’ said Danny, curious. ‘She likes gang-shags.’.
1958		(con. 1950) E. Frankel Band of Brothers 171: Boys [...] seduced their cousins, and had gang shags on the roof.
1963		in T.I. Rubin Sweet Daddy 9: A gang shack yet – musta been twenty of us.
1968		K.J. Shapiro To Abolish Children: And Other Essays 127: This chapter of the sermon was followed by a noisy and incoherent replay of a daisy chain or gang-shag.
1972		B. Rodgers Queens’ Vernacular 92: gang-bang [-shack, -shag, -shay (fr -cher in Fr coucher = to lay down) -splash] [...] 2. (camp) homosexual orgy.
1977		A. Sorrentino Organizing Against Crime 59: They may have engaged in ‘gang shag’ activities—that is, mass sexual intercourse—with girls outside the neighborhood.