It ain't over till (or until) the fat lady sings is a colloquialism and has been classified as a proverb. 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.
But this, also, is not the true origin of the exact phrase. the phrase, his utterance did result in its popularization via inspiring Bullets coach You see, “The opera ain't over till the fat lady sings,” is just a modern version of a.
As for the fat lady, I've no idea who she was or what she was singing. The correct expression is that; "the opera ain't over till the fat lady sings. Yogi Berra, also credited with "It ain't over till it's over " and "90% of the game is one half mental".