crowpeck

Australian (maybe Queensland). To make a fist with one hand, making sure your middle knuckle is protruding. Then go up behind someone and rap them hard over the back of the head with the extended knuckle.
Painful and surprising. Mostly done by jocks and thugs on dorkwads and Warcraft players.

The act of clenching a fist and pushing your index finger knuckle forward, which is then driven into someone's head. This is generally done with consent as the recipient will more than likely have lost a challenge that put him into this precarious situation.