Green’s Dictionary of Slang

1. (orig. US drugs) a drug addict who injects narcotics into the vein.

1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 123/2: main-liner. An addict who shoots narcotics into his veins.
1937		B. Dai Opium Addiction in Chicago 172: The ‘skin shooter,’ in turn may look down upon the ‘main line’ or the ‘vein shooter’.
1938		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 2 in AS XIII:3 187/2: main-line shooter, A vein-shooter.
1949		Monteleone Criminal Sl. (rev. edn).
1958		W. Motley Let No Man Write My Epitaph (1960) 170: They are all main-liners [...] They go strictly for the veins. Main-lining has a quicker effect. They are in panic for their fix.
1963		‘Geoffrey Tolhurst’ Flat 4 King’s Cross (1966) 112: Johnny was a mainliner. He injected the drug directly into his vein, and that was the end of the road for an addict. There was nowhere to go after that but to take stronger and more frequent ‘fixes’.
1964		J. Baldwin Blues for Mister Charlie 45: A junkie, a dope addict, a hophead, a mainliner — a dope fiend!
1977		C. McFadden Serial 28: There was some confusion about whether Harvey was living with a fourteen-year-old female mainliner or a male hairdresser.
2001		ONDCP Street Terms 14: Mainliner — Person who injects into the vein.
2010		K. Richards Life 259: Somwetimes I’d go to the cottage and - because they were mainliners - say, ‘Penny, is Steve still alive?’.

2. (US prison) a convict who is part of the main prison population, rather than those held in punishment cells.

1949		Monteleone Criminal Sl. (rev. edn) 151: mainliner [...] prisoners who eat in the main dining room of a jail.
1953		T. Runyon In For Life 134: A few Mainliners were definitely dangerous.
1976		M. Braly False Starts 159: At the time I was threatened with their lot the life of a homosexual prisoner was even drearier than a mainliner’s.

3. (US drugs) a hypodermic injection of narcotics into a vein.

1952

Lait & Mortimer USA Confidential 124: Many girls come to class with their arms scarred, punctures from ‘mainliners’—underworld jargon for hypodermic incisions into arteries.