The human posterior has been given a number of designations over the centuries, some not acceptable in polite company. "Prat," a slang term with only slightly indecorous overtones, has been used in reference to the backside since the 16th century. It wasn't until the 1930s, however, that falling on one's prat gave rise to the term "pratfall." The word first cropped up in the lingo of comedy theater, where a pratfall is often part of a slapstick routine. It wasn't long before we gave the word its extended sense of "blunder." Now, with "prat" rarely used as a synonym of "derriere" anymore, "pratfall" is as apt to suggest getting a pie in the face as landing bang on one's behind.