Usually used as an act of revenge. Butter Dicking somebody includes rubbing a stick of butter on your member, then rubbing your buttery member on the person (or something the person owns).
guy #1: "Theres something slimy on my car door handle"
guy #2: "Uh-oh, theres a note on your windshield that says 'You've been Butterdicked!'"
guy #1: "FML"