It ain't over till (or until) the fat lady sings is a colloquialism and has been classified as a proverb. 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 the 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 Amalie Materna played Brünnhilde during Wagner's lifetime (1876) with a winged helmet). Her aria lasts almost twenty minutes and leads directly to the end of the whole Ring Cycle. 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."
The saying has become so well known that it was the subject of an article in the journal Obesity Reviews.
It ain't over till the fat lady sings Wikipedia
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."
In the same newspaper on 26 November 2006, Steve Blow followed up the discovery by contacting Bill Morgan about the incident:
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 1978 NBA 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."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 carved in stone", a phrase meaning that the future can always be changed.