Green’s Dictionary of Slang

mainliner n.

also mainline, mainline shooter
[mainline v.]

1. [1930s+] (orig. US drugs) a drug addict who injects narcotics into the vein.

2. [1940s–70s] (US prison) a convict who is part of the main prison population, rather than those held in punishment cells.

3. [1950s] (US drugs) a hypodermic injection of narcotics into a vein.