What does "all gone pear-shaped" mean — and why is it funny?

informal, British

Meaning

Said when a plan or situation has gone badly wrong.

Where it comes from

A British phrase from the later 1900s, possibly from aviation slang, where a botched loop or manoeuvre ended up bulging and misshapen instead of round.

Why it is funny

The humor is that a pear is not even an alarming shape — it is a perfectly nice fruit. The phrase makes failure sound gentle and lopsided rather than disastrous, which is its own kind of British understatement.

Used in a sentence

"The picnic was lovely until it rained and the whole day went pear-shaped."