What is butt-rock, you ask? As usual, we've got all your answers, courtesy of Urban Dictionary:
A derogatory term for any hard-rock music.
The term comes from a nationwide advertising campaign on hard-rock radio stations in the 1990s that used the tagline "Rock. Nothing but Rock." Listeners quickly changed that to "Nothing Butt Rock." Though it refers to anything played on hard-rock stations, it commonly is used to refer to 'hair-bands' or used by people to distinguish the 'bad' butt rock from the hard rock that they like.
Example: "He sat around stoned all day listening to butt rock on the 'Wild Hare.'"
Butt-rock is that musical stank on your shoe that you can't get off. It's one part aggro noise, one part self-indulgent and whiny singer, and somehow a whole lot of douche.