Taken from the saying "It ain't over until the fat lady sings," commonly attributed to ex-Yankee ballplayer Yogi Berra.
Presumably, the "fat lady" in question is a stereotyped opera singer, replete in Wagnerian Rhinemaden garb. Ironically enough, the "fat lady" was usually singing the United States National Anthem, the traditional way to START a baseball game.
Sure, we're behind by 300 points. But it ain't over until the fat lady sings.