Definition 1: A fall on the buttocks; hence, a humiliating blunder.

Usage 1: "He slipped on a banana peel and made a perfect pratfall." The noun "prat," originally referring to the rearend, has ameliorated in Britain to a word meaning a "foolish person": "He's in trouble for forgetting his wedding anniversary -- the prat!" (thanks to Clare Morrison for the example).

Suggested usage: We should all learn from our pratfalls; they are inevitable. But the word is an underused term that means precisely "screw-up" or worse: "Citing Shakespeare as the author of the line from Gilbert and Sullivan was a right pratfall that he will just have to survive."

Etymology: Prat (archaic for "buttocks") + fall.