All the definitions before this one are in correct. Butt Rock is style of music, that came to fruition during the late 70's and early 80's. It was defined by the fact that the style of dress or 'traditional garb' of those that created and or listened to Butt Rock included tight-fitting pants that emphasized the buttocks. Meaning, pants, Jeans, etc that were so tight one could make out the outline or details of the buttocks. This was hand in hand with the sexual innuendos and double entendre's that could commonly be found in the lyrics to this style of music.
Additionally, the pants were so tight that the outline of a certain part of the male anatomy were visible. This aspect would be referred to and bring about the label of Cock Rock by those familiar with the genre, but ultimately has the same meaning as Butt Rock.
One would argue that these labels could be deemed negative or a positive, depending on your like or dislike of this particular style of music.
Band Examples: Journey, Foreigner, Ac/Dc, Def Leppard.
Ac/Dc (namely during the Bon Scott era) are a great example of butt rock.
Butt Rock is fun to listen to in small doses, or at an 80's party.
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