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1.
A highly promiscuous woman. So called because a lot of semen is deposited into her.
"Dude, that chick you're going to marry was a total walking sperm bank in college."
by Jake October 10, 2005
 
2.
A person, usually woman, who is known for having an assload of sex, all the time.
"Chris' sister is a real walking sperm bank. I'm gonna call her and see if I can make a deposit. I wonder if there's a penalty for early withdrawals."
by Johnny Christmas November 30, 2003