Ah the butt. It goes by many names, but it smells just as sweet (eat it Shakespeare). Here is a list of butt synonyms, some with etymology.
A--: first attested 1860 in nautical slang, in popular use from 1930; chiefly U.S
Batty (Jamaican slang)
Behind: Euphemistic meaning "backside of a person" is from 1786.
Boem (pronounced as "boom")
Booty: 1920s, black slang.
Bottom: Meaning "posterior of a man" is from 1794.
Bum: "Buttocks," late 14c.
Buns: The first record of buns in the sense of "male buttocks" is from 1960s.
Butt: In sense of "human posterior" it is recorded from mid-15c.
Buttocks: late 13c., probably related to O.E. buttuc "end, short piece of land."
Cheeks: "The buttocks," c.1600.
Crack in the back
Derriere: 1774, from French derrière "back part, rear," originally an adv., "behind," from Latin deretro, from Latin de "from" + retro "back."
Fanny: "Buttocks," 1920, American English, from earlier British meaning "vulva" (1879), perhaps from the name of John Cleland's heroine in the scandalous novel "Fanny Hill or Memoirs of a Woman of Pleasure" (1748). The genital sense is still the primary one outside U.S., but is not current in American English, which can have consequences when U.S. TV programs and movies air in Britain.
Hurdies (British slang)
Jacksie (British slang)
Keister: "buttocks," 1931, perhaps transferred from underworld meaning "safe, strongbox" (1914), earlier "a burglar's toolkit that can be locked" (1881).
Moon: The meaning "to flash the buttocks" is first recorded 1968, U.S. student slang, from moon n.) "buttocks" (1756), "probably from the idea of pale circularity."
Poep (like "poop")
Rump: "Hind-quarters, buttocks of an animal," c.1440, from a Scandanavian. Source (cf. Danish, Norwegian rumpe, Swedish rumpa).
South end of a north bound horse
Tush: "Backside, buttocks," 1962, an abbreviation of tochus (1914), from Yiddish tokhes, from Hebrew tahat "beneath."