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"
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.