Since there are many forms of operas, there is AFAIK not a special aria which is sung by the female protagonist at the end and there is no special meaning to it. The proverb probably stems from the fact, that a) the best opera singers were not really slim and b) there is ALWAYS a big aria of the female protagonist. If this hasn't taken place, then the opera isn't yet over.

From a more cynical viewpoint, a classic opera has three stages:
Act one) confusion and illogical plot
Act two) more confusion and complete abandon of rational conduct
Act three) Male and female protagonists die for some stupid reasons. They do this under extensive singing, which at least keeps you from commiserating.

When you have thus listened to the male protagonist dying over 25 minutes and believed that you might now be allowed to go home, your neighbour might mention that the fat lady hasn't sung yet, destroying your hopes that she might die silently.

At least twice a year this questin pops up. That's why there are many threads already in the archive which answer your question.

https://dict.leo.org/cgi-bin/dict/archiv.cgi?...

I agree with Hein -de- on all points. That's why I like listening to opera on CD or on the radio. It's easier to wallow in the music and not feel obliged to take the story too seriously. (-;

OT: Just this week there was a really nice Great Performances on PBS of the 50th anniversary concert at the Vienna State Opera:

http://www.pbs.org/wnet/gperf/shows/viennaope...

Lots of gorgeous music, no obtrusive silly plot, and those ladies who were somewhat overweight nevertheless sang beautifully. (I didn't catch who sang Aida opposite Domingo, but she was really good.)

Zu diesem Thema kann ich nur waermstens den Roman Mascerade von Terry Pratchett empfehlen!!!*gg*

Eine fat lady, eine duenne Schoenheit, die nicht singen kann, und ein moerdischer 'ghost of the opera'...

An interesting twist on this is that some people say "the fatling ain't sung yet".

Here is another explanation about the origin of this phrase:
http://www.worldwidewords.org/qa/qa-ita1.htm

Wikipedia is always helpful

http://en.wikipedia.org/wiki/Fat_lady