It Ain't Over Till the Fat Lady Sings
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A common American English proverb meaning that one should not presume to know the outcome of an event still in progress, often attributed to opera.
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Mentioned once in Kisses Sweeter Than Wine.
Kisses Sweeter Than Wine
Season 3 · Episode 5 · S03E05
“Oh, it's over, it's over. Sing, fat lady, sing.”