Green’s Dictionary of Slang

1. (drugs) a capsule containing a narcotic, usu. heroin.

1929		Spokesman-Rev. (Spokane, WA) 11 Oct. 8/4: A package of drug wrapped in paper is called a ‘deck,’ a ‘check,’ or a ‘bundle.’ If it is in a capsule it is called a ‘berry,’ a ‘bean,’ or a ‘cap’ .
1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 119/2: cap. A bindle of heroin or morphine sold in capsules which are, incidentally, often concealed by the peddler in his rolled-up shirt cuffs, heroin in one sleeve, morphine in the other.
1948		W. Burroughs letter 30 Nov. in Harris (1993) 25: I hope to rid myself of the habit in the course of this trip. Taking along a pint of P.G. and a large supply of goof balls to taper off. Also 20 caps.
1953		‘William Lee’ Junkie (1966) 45: The H caps cost three dollars each and you need at least three a day.
1958		W. Motley Let No Man Write My Epitaph (1959) 119: An ounce makes close to five hundred [...] When I fill these caps I can make a lot.
1960		C. Cooper Jr Scene (1996) 41: Only they weren’t girls, having acquired through the years [...] of practice the names: Four Caps, Six Caps and Eight Caps.
1965		C. Brown Manchild in the Promised Land (1969) 109: I had a lot of guys stealing all kinds of caps with white powder in them.
1968		L. Wolf Voices from the Love Generation 172: Three bucks for a cap.
1973		E. Bunker No Beast So Fierce 35: The jivin‘ motherfuckers give you more time for a cap of heroin than murder.
1974		V.E. Smith Jones Men 16: I don’t have but one cap of the white bag, man.
1976		R. Sabbag Snowblind (1978) 94: He could hit you with lids, caps, keys, tabs, nickel bags, blotters, buttons, spoons and everything from milligrams to boatloads.
1989		(con. 1930s) Courtwright & Des Jarlais Addicts Who Survived 192: I’ll sell them in the quarters, eights, and sixteenths, and in the small caps, and the fifty-cent packages.
1989		(con. 1982–6) T. Williams Cocaine Kids (1990) 57: One day I go out they be selling for ten dollars a cap. The next day it‘s eight dollars a cap.
1997		L. Pettiway Workin’ It 157: It’s a drug war now in the projects about this one person is selling three dollar caps, and since you got three dollar caps, our five dollar caps ain’t selling.
1999		E. Bunker Mr Blue 235: ‘Just a couple caps.’ In those days heroin, at least on the streets of LA, was sold in small number 5 gelatin capsules. A cap was still potent.
2001		ONDCP Street Terms 5: Cap — [...] a capsule of a drug.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 37/2: cap n. a capsule of marijuana oil.
2015		P. Doyle (con. 1969-1973) Big Whatever 25: A sandwich bag of double-O caps, with sparkly white powder in them [...] cocaine.
2018		W. Kramer Hard Stuff 129: The dope was sold in three-penny caps, which were small capsules of ‘mixed-jive’ dope: heroin cut with quinine to improve the rush.

2. (drugs) a capsule of LSD.

1965		P. Wylie Esquire July 44–45: cap — a capsule of LSD.
1968		‘Sl. of Watts’ in Current Sl. III:2.
1969		R.R. Lingeman Drugs from A to Z (1970) 59: cap [...] (2) currently, a capsule containing a hallucinogen.
2001		ONDCP Street Terms 5: Cap — [...] LSD.

3. (drugs) a capsule of any drug.

1968		H. Ellison ‘The Hippie-Slayer’ in Deadly Streets (1983) 107: Some were merely dropping dexy in pill or cap form.
1985		E. Leonard Glitz 202: He fixed two rum-and-cokes then, emptying a street-lude cap into Iris’s – eighty milligrams of Valium to take off her edge – and brought their drinks out to the living room.
1987		S. King Misery (1988) 124: He would have to cut down, have to duck some of the caps.
2000		R.G. Barrett Leaving Bondi (2013) [ebook] ‘I’m on my way back to a Sleaze Ball in Zetland with a thousand caps of Ebeneezer’.

4. psilocybin/psilocin.

2001

ONDCP Street Terms 5: Caps — [...] psilocybin/psilocin.

5. (drugs) crack cocaine.

1997		L. Pettiway Workin’ It 111: If they do get into a little thing it’s about a cap of crack.
2001		ONDCP Street Terms 5: Cap — Crack Cocaine.

6. (US prison) a measure of marijuana: as much as can be fitted into a small cap, e.g. of a toothpaste tube or a container of chapstick.

1992		Bentley & Corbett Prison Sl. 74: A cap may also be used as a unit of measure of marijuana. A method now being used in prison is filling the inside of a toothpaste cap with marijuana and selling this amount as a ‘box bag’ or a ten dollar ‘bag’ of marijuana.
1999		Guardian Editor 28 May 20: Cap: The marijuana that fits into a chapstick cap.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Cap: The amount of marijuana that fits into a chapstick cap.