A sound effect in a First Person Shooter called Half-Life 2, made by a rebel, indicating that a Manhack is near by.
Popurarly used in "Gmod
idiot box", a machinima series by DasBoSchitt. Shouted by "Dr. Hax", a big head copy of Dr. Breen(one of Half-Life 2 charachters), which is looking for people using cheats.
There is also a downloadable weapon made for game Garry's Mod which makes you shout "HAAX!!!" and shoot a monitor at the enemy you're pointing at.