Go pear shaped means to go wrong, often quite suddenly, unexpectedly, or badly. It is common in British English, though many other English speakers understand it too. You might use it for plans, projects, events, or situations that stop working as intended.
A popular explanation links the phrase to RAF flying training. In aerobatics, a clean loop should stay round. If the shape drops or stretches, it can look more like a pear than a circle. That visual idea helps explain why the phrase came to mean that something has gone off course or turned out badly. The exact history is not fully certain, but this origin story is widely repeated because the link is easy to understand.
In modern use, the phrase is informal and often slightly humorous. It usually suggests that a plan did not just fail, but failed in a messy or disappointing way.
- Our presentation went pear shaped when the projector stopped working.
- The deal looked promising at first, then everything went pear shaped.
You would not usually use it in very formal writing. In everyday speech, though, it is a vivid and memorable way to say that something went badly wrong.