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.

A pair of 8's in poker.

A feminine overweight person.