Green’s Dictionary of Slang

cap v.6

[cap n.4 (1)]

1. (also cap up) to transfer bulk drugs (in powder form) into capsules for sale.

[US](con. 1948) G. Mandel Flee the Angry Strangers 243: We got a little cappin to do.
[US]W. Motley Let No Man Write My Epitaph (1960) 134: If you’re a good capper and cap it yourself and sell part of it [...] You can keep your habit up.
[US]N. von Hoffman We are the People Our Parents Warned Us Against 175: Two other young men were seated around a table capping acid.
[US]‘Iceberg Slim’ Pimp 158: Let’s [...] cap up and bag up that stuff for those jokers.
[US]V.E. Smith Jones Men 179: Right now I want to cut this stuff and recap it for tomorrow.
[NZ]G. Newbold Big Huey 16: I bought a few hundred 0-size gelatine capsules from a chemist, capped the gear up, and sold it within a week.
[US]ONDCP Street Terms 5: Cap up — Transfer bulk form drugs to capsules.

2. to be given drugs for free.

[UK](con. 1950s–60s) in G. Tremlett Little Legs 193: cap being treated to drugs when you have no money.

In phrases

cap someone’s arm up (v.) [joc. var. of sense 1 above]

(US drugs) to inject a narcotic.

[US]C. Cooper Jr Scene (1996) 99: ‘I gotta [...] cap my stuff up.’ ‘You mean cap your arm up, trick!’.