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1.
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 10, 2008
 
2.
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 16, 2009