Comes from the language of 1337 or leet
in which gamers describe another as a dominating opponent. It is also a form of the word owner where the attacker is Owning the other.
d00d, taht guy is a pwn3r. check him out hes gonna go pwn that guy in the fuxoring head. B00M HEAD SH0T!
Jack's head was turned into red jello.