What does “everything's gone pear-shaped” mean?
What is the origin and meaning of "pear shaped", as in "this situation is about to go pear shaped"
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
The meaning is simply "gone wrong".
As to the origin: nobody knows.
There is a short discussion of it on the normally reliable World Wide Words here
In BE the usual meaning of "pear-shaped" is simply that something has gone wrong; there is little implication of degree. I have always understood the phrase to refer to a circle (as in the form of a wheel) becoming distorted into uselessness.