To have sex with a women when she's on her period. Red is the color of blood/Soviet Union. A submarine is very much shaped like a penis. Hence, Soviet Sub. Can be used in past and present tense.
"Dude, I Soviet Subbed her like crazy last night!"
"I was Soviet Subbing this chick, and..."
"Baby, you wanna... Soviet Sub tonight?"
"I was Soviet Subbing this chick, and..."
"Baby, you wanna... Soviet Sub tonight?"
by Arius May 1, 2008