But It's Not Over

I've often wondered where the "fat lady sings" euphemism comes from. I suppose I could google it, but that would be too easy. But the notion that nothing is "over" until a heavyset woman wearing viking armor, not unlike this MOC by Pepa Quin, comes out belting operatic tones seems really odd. What if your opera is set in ancient Mongolia? She would be completely out of place. What if it's set in modern Los Angeles? Actually, that's not much of a stretch, never mind.