Green’s Dictionary of Slang

mainline n.

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.

In phrases

bust the mainline (v.) (also burn the mainline)

[1950s+] to take an intravenous injection.

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

[1930s+] (drugs) to inject a drug.

ride the mainline (v.) (also rock the mainline)

[1950s] (drugs) to inject narcotics; to be a narcotics addict.