Interesting question… Who is the Fat Lady, and why do we have to wait for her to sing?

There are many opinions, and they differ about the origin of this colloquial proverb.

Popular belief refer to the Prima Donna, the large woman who would sing the final aria at the end of an opera, usually as she's dying.

Others claim that it’s actually a mis-quote & that the correct phrase is ‘It's not over until the fat lady sinks’ with its origins in the game of billiards. The black eight ball was commonly referred to as the "fat lady" so no matter how bad the game was for a competitor it wasn't over until the "fat lady" sank!

The message remains the same, regardless of the origin:
It means that one should not presume to know the outcome of an event which is still in progress. More specifically, the phrase is used when a situation is (or appears to be) nearing its conclusion.