In North Staffordshire - England, "Mullygrubbing" is a form of punisment, usually given by an older brother to a younger sibling. A kind of bullying. To mullygrub someone you must get the victim into a position whereby there head is clasped between your upper-legs, face forward, scalp to you. Then you rub your knuckles agressively into their hair for several seconds - even a minute or two has been known. Ask my younger sister!
Melanie you twat! For nicking my fireworks I'm going to give you a jolly good mullygrubbing!