Green’s Dictionary of Slang

mainline n.

1. (US prison, also line) the prison convict population, excluding those who are detained in punishment cells.

[US]Ersine Und. and Prison Sl.
[US]T. Runyon In For Life 117: The Mainline believed every word of it and acted accordingly.
[US]M. Braly On the Yard (2002) 12: What’s for chow on the main line tonight?
[US]Other Side of the Wall: Prisoner’s Dict. July [Internet] Mainline: The general population.

2. (orig. US) in drug uses.

(a) (also main) the vein into which an addict injects narcotics.

[US]G. Milburn ‘Convicts’ Jargon’ in AS VI:6 440: main line, n. A vein in the arm just above the elbow; the median vein.
[UK] (ref. to 1918) L. Duncan Over the Wall 21: I saw the more advanced narcotic addicts, who shot unbelievable doses of powerful heroin in the main line.
[US]‘William Lee’ Junkie (1966) 33: I began shooting in the main line to save stuff and because the immediate kick was better.
[US]Velvet Undergound ‘Sister Ray’ [lyrics] I’m searching for my mainline / I couldn’t hit it sideways.
[US]E.E. Landy Underground Dict. (1972).
[US]D.E. Miller Bk of Jargon 342: mainline: [...] 2. The vein into which heroin is injected.
Dalton Vrij ‘Tying Off’ on [Internet] Looking down at the crook of his arm where his mainline split passing as the Ganges and the Nile he wavered in his choice knowing he had one more river to cross. A speedball like any other coke issue is the ultimate crossroads of the hit or miss.

(b) (also mainline bang) an injection into the vein.

[US]Goldin et al. DAUL 133/1: Main-line bang. A hypodermic injection of a narcotic solution in the median cephalic vein in the forearm.
[US]H. Ellison Web of the City (1983) 172: He needed a fast mainline to get rid of the monkey.
[US]P. Thomas Down These Mean Streets (1970) 206: Man, a thought jumped into my mind, mainline is the best time.
[US]B. Davidson Collura (1978) 49: Collura knew there was only a brief step from skin popping [...] to mainlining.
[US](con. 1940s) Courtwright & Des Jarlais Addicts Who Survived 70: She took the spike and hit me. This was no mainline she gave me, this was a skin pop.
[US](con. early 1950s) J. Ellroy L.A. Confidential 395: I’d be on a mainline flight and Pierce, he’d be on safe old white horse up the nose.

(c) narcotics, usu. addiction.

[US]H. Simmons Corner Boy 46: Red had whispered of how terrible the mainline was.

3. see mainliner n.

In phrases

hit the mainline (v.) (also bust the mainline, hit the main vein)

(drugs) to inject a drug.

[US]D. Maurer ‘Lang. of the Und. Narcotic Addict’ Pt 2 in Lang. Und. (1981) 100/1: To bust the main-line. To inject narcotics intravenously.
[US]R. Chandler Long Good-Bye 117: Vukanich was a punk, a high-wire performer who hit the main line in his own office.
[US]P. Thomas Down These Mean Streets (1970) 206: I felt mad at me for not being satisfied to just snort or a ‘just once in a while skin-pop’. Naw, I hadda be hitting the main vein.
[US]ONDCP Street Terms 12: Hit the main line — To inject a drug.
ride the mainline (v.) (also rock the mainline)

(drugs) to inject narcotics; to be a narcotics addict.

god on Urban Dict. [Internet] rock the mainline To get ‘high’ on heroine [sic] through the means of an introvenus-needle. I.e. – Shooting up. Addict: Yo, dog... Yo got the shit to lemme rock tha mainline? Dealer: Fo’ sizzle ma nizzle, that’ll be fiddy.