Q From M-C Seminario: What’s the history behind pear-shaped?
A It’s mainly a British expression. “It’s all gone pear-shaped”, one might say with head-shaking ruefulness, in reference to an activity or project that has gone badly awry or out of control.
There are plenty of things that are literally pear-shaped, of course, such as a person’s outline, a particular cut of a diamond, or the shape of a bottle, anything in fact that is bulbous at the bottom but narrows at the top, like the pear. It isn’t immediately obvious how the literal meaning turned into the figurative one, though we do know that it started to appear in the 1960s.
A common explanation, the one accepted by Oxford Dictionaries, is that it comes from Royal Air Force slang. However, nobody there or anywhere else seems to know why. Some say that it may have been applied to the efforts of pilots to do aerobatics, such as loops. It is notoriously difficult (I am told) to get manoeuvres like this even roughly circular, and instructors would describe the resulting distorted route of the aircraft as pear-shaped.
I’ve not seen firm evidence to convince me of this explanation, which sounds a little far-fetched, but that’s the best I can do!