HAX! is a known saying by Dr.Hax from the Gmod Idiot Box series from Youtube.com. He usually says this when he sees someone hacking during any part of the episode. After he says "HAAAAXXX!!!," an old 1998 computer monitor slams into the person that he called hax on.
person 1: "Oh yeah! Cheat codes enabled!!!"
person 2: "HAX!"