Green’s Dictionary of Slang

(drugs) any form of barbiturate drug, usu. methaqualone.

1972		Smith & Gay Heroin in Perspective 206: Soaper. Quaalude.
1973		Newsweek 12 Feb. 65: An apparently insatiable market for the ‘sopers’ (catchall slang for the several brands of methaqualone).
1973		Science News 10 Mar. 152: Methaqualone—known on the streets as sopor or quaalude—is a nonbarbiturate hypnotic downer.
1981		D.E. Miller Bk of Jargon 338: sopors, soapers, soaps: Methaqualone.
1993		G. Pelecanos Nick’s Trip 75: Omajian [...] ate a soper and drank some beers at his desk.
2001		ONDCP Street Terms 20: Sopers — Depressants.