It ain't over 'til the fat lady sings
It ain't over 'til the fat lady sings is an English expression with a meaning similar to "Don't count your chickens before they hatch." It is commonly used in sports journalism, most famously by Dan Cook. This happened in April 1978 after the first game between the San Antonio Spurs and the Washington Bullets (later renamed the Washington Wizards) during the National Basketball Association playoffs. The Spurs had won the first game, but he pointed out that the series was not over yet, saying, "The opera ain't over till the fat lady sings." (The Bullets won the series, and later the NBA title).
The "fat lady" means the fat Norwegian "Brünnhilde," singing the final tragic song, wearing a Viking helmet. It can also refer to any female opera singer in general. The saying also means that "it's all over" when the fat lady sings.
- Cecil Adams, "What's the origin of "the opera ain't over till the fat lady sings?"", The Straight Dope, October 25, 1991.