To castrate someone in violence as they lay dying.
Derived from the character Percy Grimm from the novel 'Light in August', who shot Joe Christmas and then proceeded to castrate him while he was still alive.
Bob: I heard they killed Osama bin Laden
Stanly: oh yeah?, well I hope they Percy Grimmed him too!
Smiley: dude, if you don't cut that s#*% out I'm gonna Percy Grimm you!