to grind one's groin against someone's booty while dancing, usually while moving in time with each other. possibly while tapping dat ass and simulating sex.
A metaphor for: "I think you should leave. Do not pause or delay while you are leaving." Generally used when the addressed has been complaining about circumstances that the listener doesn't agree with. It comes from a time when doors were on spring hinges and if you paused in the doorway while exiting, the door would spring closed behind you and hit you on the backside.
This means the same thing as, "Beat it," or, "Get the helloutta here," or, "Yeah, the door is this way, pal."
When Wayne's job interview was going poorly and he had a fight with the interviewer, the interviewer kindly asked him to leave by saying, "Yeah, Don't let the doorhit you on the ass, buddy."