From the Urban Dictionary: "The name "butt rock" has a few possible origins. First, in the 1980s, the musicians in many hair metal bands often dressed in a "glam" style, wearing tight pants that would accentuate their butts. (This may also be the origin of the term "cock rock," which has the same connotations, as the tight pants would also accentuate the musicians' crotches.) A less flattering origin for the name is that the lead singers of these bands sounded like they were singing out of their asses. Finally, the term can generally mean that the music sounds like ass."
