A dubstep fan. Word originates from one of dubstep's prominent sonic features (i.e. wobble bass). The term is akin to "metal head" or "Dead head." (Though the latter applies to a fan of a particular band, the Grateful Dead, while "wubby" applies broadly to fans of a genre, dubstep.) It should be noted that a wubby does not inherently equate to a dubtard, though it is possible.
"Hey, I was hanging out with him the other night, and the only music he had on was dubstep."
"You should have known that, he's a wubby."
"Why doesn't this DJ play anything besides dubstep?"