Stands for "New Jersey hardcore" and means that New Jersey is, by some stretch of the imagination, a hardcore state, presumably because a lot of good bands come out of NJ and because the tool sketching "njxhc" lives there. What all the emo kids and scenesters from the Jersey suburbs like to write all over their notebooks and folders and on their Myspaces.
The headline on some poser's myspace:
someone: "New Jersey sucks!"
poser: "Forget you bro njxhc!"
normal kid: "Well Jersey is alright i guess, not really hardcore though..."