go pear-shaped

To go awry; to go wrong.

The term emerged as Royal Air Force slang, though it remained largely confined to aviation circles until it entered common usage in the 1990s. The precise origin is uncertain, and several competing theories exist to explain the transition from a sphere to a fruit. One suggestion relies on the image of a solid rectangle losing its structural integrity and slipping downward, a process described as the bottom dropping out. Similar theories involve the sagging work of potters or glassblowers, though these are often dismissed as folk etymologies.

Alternative explanations favour the catastrophic loss of internal pressure. The term may refer to the silhouette of a balloon or a football once it has been punctured and begins to deflate. Another theory suggests a physiological metaphor for those who shoulder responsibility; when such individuals descend into shirking, their shoulders are said to diminish as their bodies become pear-shaped. The Royal Air Force remains the most cited source for the expression, though the specific aerial failure that first prompted the comparison is not recorded.