Top Definition
Main vain refers to a penis. A mans main vain.
"dude, i gotta go drain the main vain" meaning he needs to urinate, piss.
by justout October 01, 2006
1 more definition
Also known as the penis. There is no muscle in the penis, the penis is erect by a massive rush of blood. Therefore another nickname for the penis is the Main Vain.
Don't use the brain, use the Main Vain!
by Butcher Jadali August 26, 2009

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