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
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the act of taking internet lingo and applying it to real life conversation

matt: I walked by a group of kids taking a test and said, "fucking scrubs."
joel: dude, you were totally mainlining.
by Taxt April 17, 2009
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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
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One who does drugs such as heroin by means of shooting up in a vein. Intraveinously
by Kyleigh January 27, 2006
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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
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to render righteousness with perfect fidelity, especially using emerging information and communication technologies. A MKULTIMATE asset commandeered from MKULTRA.
During MKULTRA, every mainline was murder; now, we are fortunate to be able to MainLine America; now, every MainLine is a Miracle.
by Medicine Owl February 8, 2023
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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
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