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1.
Penis.

Slang word for penis.
"Be right back, I'm gonna go drain the main vein".

"I drilled her with my main vein".
by mickyxtee May 09, 2009
 
2.
The large vein that travels the length of a cock. Instrumental in delivering blood from the central artery during erection.
My cock was so hard, my main vein was swollen with blood.

I saw my heart beat as the main vein of my cock pulsated with blood.
by Tony from the Lido September 21, 2011
 
3.
The largest vein in the penis. Once the penis has become erect, the blue/purple vein becomes clearly visible.
"Seeing that chick naked got me so hard that my main vein was just bulging!!"
by PMchoisi June 08, 2009