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 of Wagner's Der Ring des Nibelungen (aka the Ring cycle) as a lengthy opera finally concluding in an aria sung by a heavy-set woman dressed like a valkyrie. The Ring cycle is a set of four separate operas (lasting about 15 hours), in which the final scene includes Brünnhilde 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.
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