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.
A lot of Manhacks flying at the group
-Haacks!!!-One of rebels shout.
Chuckles the Cheat is using cheats on Counter-Strike
"Yeah!" Chuckles says.
"HAAX!!!" Pointing at Chuckles says Dr. Hax. A monitor flies at Chukles.
*Monitor collides with face, producing bloody mess*