go pear-shaped

Example: “The wedding plans went pear-shaped when the venue double-booked.”

Origin: The story usually attached to this phrase is that it began as Royal Air Force slang, describing a pilot's attempt to fly a perfect circle or loop that came out lopsided and bulging at one end. That account is repeated everywhere, and it is plausible enough as an image, but the documentary evidence does not support the date usually given with it: the earliest printed examples are only from the 1980s, four decades after the wartime period the story places it in. What can be said with confidence is that the phrase is British, that it spread widely through British English from the 1980s onward, and that its appeal lies in how precisely the image fits — a pear is not a failed shape in itself, merely a circle that has sagged out of true, which is exactly the flavor of a plan that has not collapsed outright but has quietly deformed into something no longer usable.