It ain't over till the fat lady sings
It ain't over till (or until) the fat lady sings is a colloquialism. It means that one should not presume to know the outcome of an event which is still in progress. More specifically, the phrase is used when a situation is (or appears to be) nearing its conclusion. It cautions against assuming that the current state of an event is irreversible and clearly determines how or when the event will end. The phrase is most commonly used in association with organized competitions, particularly sports.
The phrase is generally understood to be referencing the stereotypically overweight sopranos of Grand Opera. The imagery of Richard Wagner's opera cycle Der Ring des Nibelungen and its last part, Götterdämmerung, is typically the one used in depictions accompanying reference to the phrase. The "fat lady" is the valkyrie Brünnhilde, who is traditionally presented as a very buxom lady with horned helmet, spear and round shield (although Brünnhilde in fact wears a winged helmet). Her aria lasts almost twenty minutes and leads directly to the end of the opera, though the character Hagen has one final line, "Zurück vom Ring!", to sing after Brünnhilde's death, and there is also a substantial orchestral finale. As Götterdämmerung is about the end of the world (or at least the world of the Norse gods), in a very significant way "it is [all] over when the fat lady sings."
Ralph Carpenter 
The first recorded use appeared in the Dallas Morning News on 10 March 1976:
Despite his obvious allegiance to the Red Raiders, Texas Tech sports information director Ralph Carpenter was the picture of professional objectivity when the Aggies rallied for a 72–72 tie late in the SWC tournament finals. "Hey, Ralph," said Bill Morgan, "this... is going to be a tight one after all." "Right", said Ralph, "the opera ain’t over until the fat lady sings."
"Bill vividly remembers the comment and the uproar it caused throughout the press box. He always assumed it was coined on the spot. 'Oh, yeah, it was vintage Carpenter. He was one of the world’s funniest guys,' said Bill, a contender for that title himself."
The 1976 use of the phrase was discovered by Fred R. Shapiro, who published it in The Yale Book of Quotations. It had previously been attributed to sportswriter and broadcaster Dan Cook, who used the phrase after the first basketball game between the San Antonio Spurs and the Washington Bullets (now the Washington Wizards) during the 1977–1978 National Basketball Association playoffs. Cook used the line to illustrate that while the Spurs had won once, the series was not over yet. Shapiro called this a notable example of misattribution.
Phrases with similar meanings 
- "It ain't over till it's over", a phrase popularized by baseball player Yogi Berra.
- "Don't count your chickens before they hatch", a well-known saying which originated in the 16th century.
- "Nothing is set in stone," a phrase meaning that the future can always be changed.
- Cecil Adams, "What's the origin of "the opera ain't over till the fat lady sings?"", The Straight Dope, October 25, 1991.
- Fred R. Shapiro, ed. (2006). The Yale Book of Quotations. Yale University Press. p. xix. ISBN 978-0-300-10798-2.