The expression ass refers to derriere which is often referred to with more colloquial expressions like ass, booty, or buns. On the erotic market an entire branch has established itself which is dedicated to the sexual appeal of the behind only. The bottom is a very intimate body part which is very close to the primary sexual organs. Besides its erotic appeal, the behind is also associated with a certain shame which is also the reason why it has to remain covered, just like all sexual organs, in most cultures. Especially the media hawk the term ass. There are for instance numerous musical oeuvres in which phrases like shake your ass can be heard with lightly dressed women underscoring the theme of the song with their dance moves.