"buttock," 1560s, criminals' slang, of unknown origin. Later in U.S. criminal slang, "hip pocket" (1914), and in British slang "contemptible person" (1968).
Entries linking to prat
pratfall n.
"a comedy fall," by 1930, said to be a word from burlesque or vaudeville theater, from prat "buttock" + fall (n.). "Chiefly N. Amer. slang" [OED]. As a verb from 1940.