Green’s Dictionary of Slang

1. (drugs, also channel line) the vein into which a drug is injected.

1938		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 2 in AS XIII:3 182/2: channel. A vein or main-line into which a vein-shooter injects the narcotic solution.
1949		Monteleone Criminal Sl. (rev. edn) 47: channel line The vein into which injections are made by addicts.
1953		Anslinger & Tompkins Traffic In Narcotics 306: channel. The vein into which injections of a drug are made.
1960, 1975		Wentworth & Flexner DAS.
1969		R.R. Lingeman Drugs from A to Z (1970).
2001		ONDCP Street Terms 5: Channel — Vein into which a drug is injected.

2. (S.Afr. prison) an inmate courier of illicit and smuggled goods.

1949

H.C. Bosman Cold Stone Jug (1981) II 48: The warder would hand the tobacco to the ‘channel’ – some long-timer. [...] This convict would again take half the tobacco for himself and pass on the remainder.