When you have sex. It comes from the act of "zipping" your clothing off before you begin having sex.
"Hey Mark, Willie was zipping down in the Holiday Inn with this one chick he picked up off the street."
"I couldn't sleep last night because E.J. zipped down with this one chick in the other room."
"Don't go in the room, I think E-Jay is zipping down with some girl from last night's party!"