It Ain’t Over Till the Fat Lady Sings

What does the idiom “It Ain’t Over Till the Fat Lady Sings” mean?

Definition:

Do not give up too soon; things may improve.

For example:

“Yes, we seem to be losing the game, but it ain’t over ’til the fat lady sings.”

Notes:

You also hear “The opera’s not over till the fat lady sings.”