When the butt crack of a basketball is dragged down the face of a defender, or team mate as he descends from a dunk.
Did you see the mud pack that Kobe Pryant got when Derrick Rose dunked on him?!!!
It's what happens when you get your asshole invaded.
If you got fucked in your ass then you got your mud packed. The person that hit you in your shitter spitter is a mud packer.