What is Main Vein?
1.
Penis.
Slang word for penis.
"Be right back, I'm gonna go drain the main vein".
"I drilled her with my main vein".
See
2.
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!!"
See