"It ain’t over ‘til the fat lady sings."
The famous proverb is actually “the opera ain’t over ‘til the fat lady sings.”
It is a reference to buxom Brunhilde’s 10-minute aria at the end of Wagner’s Ring cycle operas.
Although the expression is commonly used in a sports context, there was already a similar expression in use in the US South: "Church ain't over till the fat lady sings". This was a reference to the choir singing the final hymn, perhaps with a soloist.
The use of the phrase in relation to sports first occurred in 978 when writer broadcaster Dan Cook used it to indicate that the basketball playoffs were not yet over for the Washington Bullets, despite being down three games to one to the San Antonio Spurs. Bullets coach Dick Motta heard the broadcast and used it to address his team. The expression became the team’s rallying cry and they went on to win the championship.