"What does ba mean?"

Seth has been a chatterbox recently. He has a bunch of words that we can identify - Mom, Dad, Ball, Bubbles, Bye Bye, and Balloon. But then he has a bunch of other things that he says with some regularity that we still have not figured out what he is referring to.

Today he started saying "ba" over and over again. When asked the very important question: "What does ba mean?" Seth's reply is always the same: "Ba."