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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?"
by Arius April 30, 2008
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