To inject a drug, usually heroin, into a vein. Also used to refer to the drug itself.
"That chick was mainlining heroin daily till she OD'd."

From Queensryche - Operation Mindcrime:

"Had a habit doing mainline
Watch the dragon burn
No regrets, you've got no goals
Nothing more to learn"
by eidolon July 18, 2004
(n) The front-running leader(s) of an industry; not the imitation spin-offs. Reserved for the select few whom others try to imitate albeit never successfully. If you are not mainline you are hopeless and should probably just quit what you do, now.

(adj) Describing an action that would only be done by someone/something that is mainline.
Guy 1: Why does that guy drive such a shitty old car if he's a lawyer?

Guy 2: He must not be mainline. If he were, he would drive a BMW 5 Series.

Guy 1: Lebron James is overrated.

Guy 2: Seriously?! He is the most mainline basketball player of this generation.
by Reacharoundartist May 22, 2010
the intravenous injection of a drug. which means using a needle to get a drug into your vein, to absorb it through your bloodstream. a lot of drugs can be taken this way, the most commonly associated drug being heroin.
Layne accidentally overdosed by mainlining heroin.
by two blue lights November 11, 2008
One who does drugs such as heroin by means of shooting up in a vein. Intraveinously
by Kyleigh January 27, 2006
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 26, 2006
To watch a large amount of a geeky television show, episode after episode, for a long period of time. A season of Buffy in two days, for example.

Similar to marathoning movies or chain smoking, but more geeky.
"I just mainlined the first 3 seasons of Angel"

"I'm going to go home and mainline the LOTR movies."

"Wheres Ferg at?"
"Oh, hes at home, sitting in the dark, mainlining Simpsons episodes"
by Anna Heavens January 27, 2009
The act of binge-watching or binge-listening all episodes of a podcast or all episodes in a season of a TV show without breaking for sleep.
Mackey: Where were you last night?
Buddy: Aw, man, I was mainlining "Sense8." After the second episode, I was addicted!
by GypsyBowl July 5, 2015