Pear-shaped

In the sense of something going awry or out of control, is a relatively modern expression dating from the mid-20th century. It obviously has something to do with the shape of a pear, bulbous at the bottom and narrow at the top, and the explanation for its origin, the one accepted by the OED, is that it derives from Royal Air Force slang. That is as far as the OED ventures on the subject but that has not deterred further speculation from other sources. Following up on the RAF slang story, some say the expression comes from aerobatics. Perfect, circular loops are notoriously difficult to perform and the resulting misshapen ones are described by RAF flight instructors as having gone pear-shaped. Others maintain the expression comes from ballooning where partially deflated balloons go pear-shaped. However, neither the OED nor the RAF seems willing to confirm these explanations. Until more convincing evidence comes along, RAF slang from the mid-20th century will have to suffice.