An act in which the male places his testicles on a girl's breast and then proceeds to bounce them off her chin. This is known as the serve. The balls then go to the other titty and a rally is started. The tennis match continues this way until either partner grows tired and then milkshakes are consumed.
"Hey Tabitha, are you up for a game of titty tennis? I promise I won't serve it too hard"
"My chin is still sore from the other day. Are those regulation size balls?"