Not the best English but that's how it's spoken.
Ain't - a very common abbreviation of is not.
But what does this mean?
It means something is not over until the very end.
It's a quote of a theatre and film performer from the 1900's called Groucho Marx and refers to the custom of Wagner operas ending with a big finale from a large lady.
Here's Groucho in action - you'll need to listen carefully - he speaks quickly and his jokes are sharper than a carving knife. Usually, he's trying to con money out of people.
