Green’s Dictionary of Slang

1. (drugs) one-eighth of an ounce of a drug, usu. a narcotic.

1937		B. Dai Opium Addiction in Chicago.
1938		D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 101/2: eighth-piece. See piece.
a.1964		‘Dumbo the Junkie’ in D. Wepman et al. Life (1976) 91: So I shot an eighth of shit and went back to bed.
1971		E.E. Landy Underground Dict. (1972).
1981		D.E. Miller Bk of Jargon 341: eighth: Approximately one-eighth ounce of heroin prepared for sale.
1989		(con. late 1940s) Courtwright & Des Jarlais Addicts Who Survived 134: I used to pay twenty dollars an eighth for it.
1992		in R. Graef Living Dangerously 46: Draw, hash, pot or whatever, £15 for an eighth, which is 3.5 grams.
2000		N. Griffiths Grits 408: Margaret’s staying in with an eighth and a video.
2018		Twitter 25 May 🌐 Eighths are for smoking, not for denying women the right to healthcare.

2. (drugs) one-eighth of a gram of a drug, usu. a narcotic.

1989		(con. 1982–6) T. Williams Cocaine Kids (1990) 35: Rarely is any one member [...] allowed to distribute more than 250 grams – a quarter kilo – at one time. A crew leader like Chillie is given half that, an ‘eighth,’ each week.
2000		W.T. Vollmann Royal Family 247: China white. One eight G.
2015		L. McInerney Glorious Heresies 221: The weight of his psychosis dwarfed a piddling fucking eighth.