It is an expression of misery.
It started when a man was forced to go to the opera with his wife.
45 minutes into the opera the man was ready to plotz. Looking at his watch every minute,and after what seemed like hours, he turned to his wife and said he could not take it anymore, and was begging to know if they can leave now.
His wife calmly turned to him and said, “We are not leaving. It aint over until the fat lady over there sings”. At that point, the man shot himself.
True story.
The irony is that this story was subsequently made into an opera.