chandra:
I think that the word buttocks,though old fashioned.is the most appropriate,denoting the mass of gluteus maximus muscles which is a characteristic of human species. In no other species they are so well developed as they serve the purpose of cushions for sitting. For me,other words like backside,bottom,posterior etc.do not appear to be anatomically precise. Words like bum.arse.derriere etc. are more colloquial! In ancient India,heavy buttocks,along with breasts.were looked at as objects of beauty and never as a terget for castigation.Sculptures in temples like Khajuraho and Konark, godesses have been depicted with huge round buttocks. In fact,in the Hindu sacred text, Soundrya Lahiri there are a few lines praising the beauty of the hindquarters of Devi.
Well, the word bottom is one with numerous meanings (ambiguous), RE the one involving human anatomy it would be a synonym for buttocks--as would posterior, backside, behind, rump, rear, rear end, fanny (in North America), hind end, hiney, buns, etc. --so used in that context I wouldn't say that it lacks anatomical precision, although perhaps it doesn't have the more clinical connotation that terms like buttocks, posterior and hindquarters (which is rarely used in conversation) would possess.
In one discussion thread on this board there was a mention of the Greek goddess of love often being referred to as "Aphrodite Callipygius," i.e. "Aphrodite of the Beautiful Buttocks," so I'm figuring that many earlier human cultures had a fascination with attractive feminine derrieres--as of course does much of modern civilization as well.
I certainly do, although for some strange reason I like my ladies strong-armed too, like "Rosie the Riveter"... --C.K.