Originally Posted by onesnowyowl

It means you're using some kind of "hack" (an actual hack, a bot, a cheat or whatever) to be better at the game than your opponent...people usually say "wtf hax" to say you're better than they are or have some kind of unfair advantage over themselves and they don't know what it is.

In WoW this sometimes also happens due to bugs in the game. I've had buffs last longer than they were supposed to, including one that made me nearly invincible compared to my size...that was nice.