Green’s Dictionary of Slang

(drugs) one 16th of an ounce of a given drug.

1952		Hal Ellson Golden Spike 13: I bought us a sixteenth in a package.
1958		W. Motley Let No Man Write My Epitaph (1960) 163: Some guys go out [...] and buy a sixteenth. They shoot it up.
1969		R.R. Lingeman Drugs from A to Z (1970).
1971		E.E. Landy Underground Dict. (1972).
1989		Courtwright & Des Jarlais Addicts Who Survived 181: I remember you could get a vial of H for two dollars. It was about what we would call a ‘sixteenth’ today.
1993		E. Richards Cocaine True 78: A guy came up in a car and handed me a quarter-ounce. No, a sixteenth.
2000		N. Barlay Crumple Zone 197: A neat row of sixteenths in clingfilm, wraps cut out of a magazine.