Green’s Dictionary of Slang

boy-girl n.2

[boy n.2 (5a) + girl n.2 (1)]

(US drugs) a mixture of heroin and cocaine, usu. as an injection but sometimes inhaled.

[US]Time 6 July 59: A few [...] mix their C with heroin in a process called ‘speedballing’ or ‘boy-girl.’ [HDAS].