Common internet spelling of Hacker, mostly on Counter Strike. It origins are unknown, yet it is surprisingly not any faster to write than hacker. A Haxxor is someone who hacks or cheats the game giving them cheap or unfair abilities. Some good players who spend their living playing video games are also called haXXors because they are too damn good.
BoB:OMFG teh haXXor is just got head shots on our whole team!