backseat changeup 

A backseat changeup is what happens after a double date when you're all so drunk you get into the wrong car's back seat and fuck some dope you don't even know.

Jeff) Oh hey Bill, how did you and Caron like our double date last night?
Bill) I thought it was a lot of fun, I'm concerned that I pulled a backseat changeup with Betty though.
Jeff)

backseat changeup by Whatsaspacebar? December 22, 2008