go pear-shaped

What is the origin of the idiom "go pear-shaped" and when to use it?

The idiom "go pear-shaped" is typically used in British English to describe a situation that has gone wrong or become a failure. The origin of this phrase is unclear, but it is believed to have originated in the Royal Air Force (RAF) in the 1940s, where pilots would use the phrase to describe a flight that had gone awry and ended up in an unplanned, pear-shaped flight path. Over time, the phrase has come to be used more generally to describe any situation that has gone wrong or become a mess.