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.