In opera, audiences would start to leave in between acts thinking it was over. Many operas supposedly announced the end when a blonde women with long braids and a Viking helmet with horns would sing.
This could be self explanitory. Actually, all you need to do is look at just about any SUV you see driving by, and you will see her in the drivers seat.
Who is that driving in from of me, blocking my view? Oh, it must be SUV Fat Lady driving her SUV. Where is her husband? Well her husband is at work, busting his butt so his sponge wife will have enough money to spend.
Taken from the saying "It ain't over until the fat lady sings," commonly attributed to ex-Yankee ballplayer Yogi Berra.
Presumably, the "fat lady" in question is a stereotyped opera singer, replete in Wagnerian Rhinemaden garb. Ironically enough, the "fat lady" was usually singing the United States National Anthem, the traditional way to START a baseball game.
Sure, we're behind by 300points. But it ain't over until the fat lady sings.
The way a fat lady makes her way quickly, in a dance-like fashion, through a crowded room to get to the buffet.
At the wedding reception, ____ did the fat lady shuffle when she brought her clenched fists up to her arm pits, smiled and sort of jogged across the dance floor, trying to be cute as she made her way to the buffet...again.