Top Definition
If a friend of you hooks up with a person you introduced him with, then he is in your debt. He owes you Sperm Tax during the time he is together with the person you introduced him to, whether only for a night or during the longterm relationship. Sperm Tax is a concept tax and in no way related to actual money or quantities of real sperm. Mostly Sperm Tax is ‘paid’ by small favors or best by finding a partner for the person.
I heard Chris is still together with Amy. He owes me a lot of sperm tax.
by glaif July 22, 2010

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