(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.
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"
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"
A person's main significant other, even though they may be dating several people at once.
"I got a lot of chicks, I ain't gotta lie. But you keep it real, so you my mainline." - Ray Vicks