Green’s Dictionary of Slang

(drugs) to smoke, sniff or inject a mixture of heroin and cocaine or morphine and cocaine; thus speedballing n.

1968		N. Heard Howard Street 179: When a cat mixes some heroin and cocaine together and shoots it [...] That’s what’s called speedballin’.
1989		(con. 1940s) Courtwright & Des Jarlais Addicts Who Survived 134: Sometimes I’d speedball, using coke and morphine together.
1999		Guardian Rev. 16 July 3: Tom finds a naked hooker called Mandy who has speedballed out.
2001		ONDCP Street Terms 20: Speedballing — To shoot up or smoke a mixture of cocaine and heroin; ecstasy mixed with ketamine.