it ain't/it's not over till the fat lady sings

it ain’t/it’s not ˌover till the fat lady ˈsings

(saying) used near the end of a competition, race, etc. to say that it is not finished yet, especially when you think that the person/team who is losing still have a chance to win: Some people think he’s already lost the election, but it ain’t over till the fat lady sings, you know!This refers to the fact that some operas end with a woman singing before she dies.
See also: fat, lady, not, over, sing, till