Hax0r1! is an achievement/trophy in Deus Ex: Human Revolution. It is earned by hacking 50 devices in the game successfully in the same playthrough. You will also pick up the First Hack achievement during the course of attaining Hax0r1!.
There are hundreds of devices in the game available for you to hack, including, but not limited to, computers, security terminals and alarm panels. It is beneficial to invest your Praxis points in hacking augments such as Hacking: Capture and Hacking: Stealth to enable you to hack higher level devices and with more ease respectively. This is especially useful for players who prefer not to have firefights, or are striving for the Pacifist achievement.
The name of this achievement is probably a reference to the first Deus Ex cheat codes.