1) the act of putting ones orifice (specifically the mouth) up against the crack of the person you love. This is usually referred to german bondage porn.
2) An alternate explanation is usually a sarcastic put down to anyone hoping for something.
1) Q:will you kiss my ass?
A: WHY SURE HERR OBEINHOUSER i shall kiss the shit off your crack.
2) What the fuck were you hoping for? KISS MY ASS.