What does the acronym Yowsas stand for?

It's not an acronym, it's another way of saying 'Yowza!' - an exclamation of surprise, or a way of saying "yes" (corruption of "yes, sir"). It is somewhat archaic. It was the trade mark of a Big Band leader of the 1930's named Ben Bernie. He would always sign on by saying, "Yowzah, yowzah, yowzah, this is the old maestro, and all the lads".