gapper, n.² — Green’s Dictionary of Slang

gapper n.²

[gap v. (2); the immediate effect of such a drug is to make one sleepy, thus one’s mouth ‘gapes’ open; yawning is also a symptom of withdrawal]

1. a narcotics user in the first stages of withdrawal.

1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 102/2: gapper. An addict in need of narcotics.
1953		Anslinger & Tompkins Traffic In Narcotics 308: gapper. An addict who is desperate for drugs.
1959		J.E. Schmidt Narcotics Lingo and Lore.

2. (US black) a narcotic drug, usu. heroin.

1971

D. Goines Dopefiend (1991) 23: You think you can toss in a little gapper for me . . . ?