It’s not over until the fat lady sings
Definition 1 of 1
Pro Tip 1/3Metaphoric Origin
Originally from opera where a 'fat lady' often signaled the end; now it means things can change. 
They were down by 10, but it's not over until the fat lady sings.
Pro Tip 2/3Avoid Misinterpretation
Make sure people understand it is a humorous expression, not a comment on someone's weight. 
I think we'll lose this game—no offense to anyone, you know, it's not over until the fat lady sings.
Pro Tip 3/3Colloquial Use
Best used in informal settings, too casual for formal writings or speeches. 
We still have a chance to win, remember, it's not over until the fat lady sings.