I am sorry I don't know the derivation of this phrase, but I think I can show my opinion on why it uses "fat lady" but not "slim lady" or some else. It does have some connection with opera, and in my memories, the lady who sings well and high-pitchedly is usually fat in old operas. And moreover, customarily the last program in an opera will be the best one to show the end and lead to the climax of the opera. Do you think so?

Jessie Xu

Jessie