1. [1930s+] (US prison, also line) the prison convict population, excluding those who are detained in punishment cells.
2. (orig. US) in drug uses.
(a) [1930s+] (also main) the vein into which an addict injects narcotics.
(b) [1940s+] (also mainline bang) an injection into the vein.
(c) [1950s] narcotics, usu. addiction.
3. see mainliner n.
[1950s+] to take an intravenous injection.
[1930s+] (drugs) to inject a drug.
[1950s] (drugs) to inject narcotics; to be a narcotics addict.