Originally it was a label for post grunge bands like Nickelback with perceived lack of talent but high commercial success, but IME these days it's often applied to hair metal bands and/or glam bands and the like going all the way back to at least the mid eighties too. Style over substance is the gist of it, I reckon. Or maybe just "music I don't like."![]()
I could be wrong but I think the name comes from the kinds of radio stations that play stuff like Nickelback. "WKJR! Nothing BUT ROCK!!!!" => "butt rock."