"It's not over until the fat lady sings" is an idiom, it is something that you say when someone is losing a game or competition, but you think there is still a chance they might win. There is nothing dirty about it at all. The origin of this idiom refers to an actual "fat lady" named Kate Smith. She was a large woman who was very popular as a singer in the 1930's to 1950's (Irving Berlin wrote God Bless America for her) In the 1950's, Kate had her own TV show, which ended right before the station went off the air for the night - thus, the TV broadcast day was not over until this particular fat lady sang her closing song.

If you call some one in particular a FAT LADY than that would be rude, or impolite. however this statement is not rude. The "fat lady" refers to opera, and i suppose the last act was all ways a larger set woman. It has manifested into a statement which means "this isnt over yet"

Not dirty at all. In the old days of opera singers, all the good female singers were fat. Many operas ended with a solo from the main female singer. Such is the origin of the phrase. I have a hunch the Americans stole the phrase and applied it to Kate Smith. They stole a whole language, actually. LOL.