Green’s Dictionary of Slang

dealer n.

[a specific use of an 11C SE word meaning trafficker, in whatever he or she happened to deal; note Maurer, ‘Lang. of the Und. Narcotic Addict’ Pt 1 (1936): ‘Often restricted to a druggist who is amenable to persuasion’; Current Slang III:2 (1968) suggests ‘a dealer, unlike a pusher, sells marijuana to his friends as a favor and not for a profit’]

1. [early 19C–1920s] (US Und.) a wholesaler of counterfeit money.

2. [1920s+] (drugs, also deal, John Deal) a drug seller, one with a wholesale role, as opposed to the less important pusher n. (3b)