Pear-shaped is a metaphorical term with several meanings, all in reference to the shape of a pear, i.e. tapering towards the top and rounded at the bottom.

The oldest usage from the Oxford English Dictionary (OED) is the most literal, a 1731 reference in a gardening dictionary by horticulturist Alan Van Dyke comparing the shape of the fruits of the cashew and avocado to that of a pear.

The comparison is less literal when the term is applied to people, where it means wide at the hips, a use that goes back to at least 1815, and one that can have either positive connotations (as in Venus figurines) or negative, depending upon the context.

In the 20th century, another, more abstract use of the term evolved. When said of someone's voice, "pear-shaped" means rich and sonorous. The OED dates this use to 1925.

The third meaning is mostly limited to the United Kingdom, Ireland and Australasia. It describes a situation that went awry, perhaps horribly wrong. A failed bank robbery, for example, could be said to have "gone pear-shaped". Less well known in the US it generated some media interest when British politician Margaret Thatcher used the phrase in front of the world's press at one of her first meetings with President Ronald Reagan, with many reporters being unsure of the meaning of the term.

The origin for this use of the term is in dispute. The OED cites its origin as within the Royal Air Force; as of 2003 the earliest citation there is a quote in the 1983 book Air War South Atlantic (ISBN 0-283-99035-X). Others date it to the RAF in the 1940s, from pilots attempting to perform aerial manoeuvres such as loops. These are difficult to form perfectly, and are usually noticeably distorted—i.e., pear-shaped.

Other theories include:

• If the original plan was to be visualised as a perfect circle, then the failed execution might be pictured as a distorted figure, hence "pear-shaped".
• Some aircraft engines become distorted (pear-shaped) in the event of failure.
• Early biplane aircraft buckled into a pear-shape when they crashed, especially stalling on take-off.
• The phrase refers to the shape of a gas balloon when it loses pressure. Gas balloons are spherical due to aerostatic pressure, but when they leak the gas rises to the top of the balloon and the neck bunches up, causing the balloon to look like an upside-down pear. The phrase hails from Victorian England when gas balloons first became popular.
• In glass blowing it describes a failed circular blown vessel. If over heated the glass becomes too fluid and distorts under gravity as it cools, resulting in a pear-shaped vessel. This was particularly important with early experiments with cathode ray tubes, where creating a large spherical glass vessel was necessary. Blowing such an object was a challenge and often 'went pear shaped'.
• It may be a mechanical engineering term: White metal bearings (large stationary engines and the like) when worn become "pear shaped" sometimes, due to wear and tear. It was also used to indicate poor workmanship in the manufacturing.