It ain't over 'til the fat lady sings is a proverb, essentially meaning that one shouldn't assume the outcome of some activity (frequently a sports game) until it has actually finished.
This phrase in turn refers to the impression by many that at the end of every opera, an aria is sung by a heavy-set woman dressed like a valkyrie. A famous example of this is Wagner's Der Ring des Nibelungen (aka the Ring cycle). This is a set of 4 separate operas (lasting about 15 hours), in which the final scene includes Brünnhilde (a very large Valkyrie) singing, and then riding onto Siegfried's funeral pyre. The set collapses and the entire cycle ends up in the Rhine river, where it started. The "fat lady" is often illustrated with a horned helmet, a spear, possibly a shield, and possibly blond braids (to suggest Scandinavian ancestry).
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