One who injects drugs into their bloodstream with a needle.
Ohmygod, he's not only a mainliner, he reuses his works! Do not exchange any bodily fluids with him no matter what.
by mjrn February 25, 2006
One who does drugs such as heroin by means of shooting up in a vein. Intraveinously
Hes a mainliner druggie
by Kyleigh January 26, 2006