it isn't over till/until the fat lady sings

The final outcome cannot be assumed or determined until a given situation, event, etc., is completely finished. The phrase refers to the stereotypically stout female sopranos of the opera, particularly the Valkyrie Brünnehilde, whose aria concludes Richard Wagner's opera cycle Der Ring des Nibelungen. It is often used in reference to organized competitions, such as sporting events, political elections, etc. A: "We're down by six points with only two minutes remaining. There's no way we can win now!" B: "Hey, it isn't over till the fat lady sings!" It's not looking likely that the senator will be elected to another term, but it isn't over until the fat lady sings.

If you say it isn't over until the fat lady sings, you mean that something, especially the result of a contest, is not certain and there is still time for the situation to change. If they win tomorrow's match, there's still a very small chance they might get through. You know what they say, it isn't over until the fat lady sings. Note: This expression can be varied, for instance using ain't instead of isn't and till instead of until. While saying it wasn't over till the fat lady sings, Mr Sidebottom admitted it wasn't looking good for him. Jack tells his teammates not to give up because it ain't over till the fat lady sings. Note: The origin of this expression is uncertain but it may refer humorously to the fact that in the final part of an opera there is often a piece sung by the heroine before her death, a part often played by a large soprano singer.