Here's a breakdown:
* The origin: The expression is commonly attributed to American sportscaster Dan Cook. He is said to have used it during a 1978 broadcast of the NCAA men's basketball championship game, where the game was extremely close and tense.
* The opera connection: In opera, the final act often features a grand aria sung by a leading female character. This aria serves as the climax of the performance and often seals the fate of the characters. So, it was understood that the opera wasn't truly over until the "fat lady" (a character who likely had a large role) had sung her final aria.
* The evolution: Over time, the expression has become a widely used idiom to refer to any situation where it's too early to declare a winner or outcome. The "fat lady" part has even been replaced with other images, like "until the last whistle blows" or "until the final bell rings," all conveying the same meaning of uncertainty.
While Cook may have popularized the expression, its roots are firmly planted in the world of opera.