"The fat lady sings" was an Earth colloquialism. The full statement suggested that one shouldn't presume the outcome while an event was still in progress.
When talking to Zotoh Zhaan and suggesting that she should have lied to him about Maldis, John Crichton turned it slightly on its head stating "Stick a fork in him! He's done! He's over! The fat lady is singing." ("Picture If You Will")