Generally meaning one who hacks fairly well and often.
Can be used when someone is of 1337 (leet (elite)) skill in a game or when someone is incredibly lucky and therefore must have cheated or "hacked."
When being called a h4x0r, it is usually taken as a compliment by gamers unless used in the middle of a cuss-filled rant about how that should not happened.
Angry Gamer: *Killed in game* WHAT THE HACKS?!?!?! Fucking h4x0r!