Green’s Dictionary of Slang

copman n.2

[cop v. (2a)]

(US drugs) a drug dealer; see cit. 1989.

[US]R.R. Lingeman Drugs from A to Z (1970).
[US]Maurer & Vogel Narcotics and Narcotic Addiction in Maurer Lang. Und. (1981).
[US](con. 1982–6) T. Williams Cocaine Kids (1990) 136: copman drug dealer who has fallen from grace but still has access to high-level suppliers – but can only get drugs for cash, not on consignment.