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
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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