The act of trying to enter into the middle of a conversation two people are having. Usually saying "what happened?" or "who are we talking about?"
One can be let known he/she is side busting by one of the two people in the original conversation saying "Ouch, my side!"
Man, John is the worst at side busting, hes done it about three times today.
If one more person side busts into this conversation my hips are going to shatter.