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.