Green’s Dictionary of Slang

1. a glass of tea or relatively non-intoxicating alcohol, drunk by bar hostesses .

1957

N. Algren ‘G-String Gomorrah’ in Entrapment (2009) 198: The girls in their gowns will be playing booth and bar stool for ‘spitbacks’ — shot glasses of tea and sherry, but never of hard liquor; her work is to get the mark loaded, not herself.

2. (US drugs) regurgitated methadone, saved from a heroin user’s dose (which is supposedly swallowed when handed out at a clinic) and sold to fellow-users.

1999		J. Maple Crime Fighter 45: [P]lenty of customers were willing to pay for the [...] high of ‘spitback’—an orange juice-like concoction that the methadone users manufactured by spitting out their daily ration from the clinics.
2003		J. Lethem Fortress of Solitude 446: A spitback sack was a parcel of liquid drugs, Methadone, smuggled from the dispensary [...] by the method of concealing a few fingers of a Saran-wrap glove in the throat or the cheek to catch the spitback.
2004		E. Conlon Blue Blood 417: The junkies sold ‘spitback’ from litle plastic juice bottles. (Methadone must be consumed on site at the clinic, and spitback is just what it sounds like.).