When you are really excited or happy and you begin to start tapping a beat on your stomach. Usually the beat is just random but can also be found as a recently heard song.
Your hanging with your mates and are having a great time. Without even realising, your joy builds up so much that you have a sudden urge to start slapping a beat onto your tummy. Most of the time you don't even realise your doing it so therefore you are giving yourself a Tummy Drummy.