It means something went wrong or didn’t go to plan. In British English, “it’s gone pear shaped” is an informal way to say a situation has failed, turned messy, or ended badly.

Example

  • “We were supposed to finish the project today, but it’s gone pear shaped.”

Tone

People often use it in a slightly light, conversational way, not usually for very serious situations.

In plain English

  • Good: things are working.
  • Gone pear shaped: things have gone off track or collapsed.

TL;DR: It means things have gone wrong.