Many think this is a reletively new abbreiviation of the word "hardcore," but infact it's an abbreiviation of the word "hardxcore." This is when someone is so "hardcore" they are straight edge aswell.
The "X" in the word symbolises straight edge because it has straight edges.
Guy 1: OMFG that scene is so hardcore, with his tee's and hightops.
Guy 2: Yeah, I hear he's a vegan and doesn't even smoke.
Guy 1: He must be HXC