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In the most basic of explanations, it means to cause a sex-induced coma. The goal is to fuck someone so well, that they fall to sleep and are late for work on a Monday when it's only Saturday.
MAN CALLING HIS GIRLFRIEND: "I hope you got another sick day, cause I'm about to knock it out."

GIRLFRIEND: "I'm on my way!" (doing 100mph)
by Moody G July 26, 2008
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