What does the expression 'It ain't over till the fat lady sings' have to do with sports?

Casey Stengel, the legendary Baseball coach, used this expression among other malapropisms during his career. It references the ending aria in Opera at a time when all sopranos in opera were fat. The last song in an opera was sung by a fat lady...then the opera was over.

Casey Stengel apparently had been to an opera and didn't understand it...but the saying lives on to this day.

...actually, Casey Stengel coined the phrase, "It ain't over 'til it's over." The "fat lady" phrase is attributed to sportscaster Dan Cook who first used it during a Bullets game back in '78. Oops, my bad... it was actually Yogi Berra that coined the "ain't over 'til it's over" phrase. Casey Stengel had nothing to do with either phrase, but he & Yogi are both famous for their "isms".