Shake off a fastball 

It means going number 2 in the bathroom. In baseball, when a pitcher wants to throw a fastball, the catcher puts one finger down. If he wants to throw a curveball, he shakes his head to the fastball and the catcher puts 2 fingers down for a curveball. Also known as "calling for a curveball".

I need to go to the restroom and shake off a fastball

Shake off a fastball by CPAbel February 4, 2010