Standing in a corridor (at school/work etc) with your back against a wall and using your bum to push yourself away from the wall, and using it to cushion your return to the wall. Can be used to pass the time while waiting for a class to begin/person to arrive. Is closely related to bum walling.
1) Hey dude, you are so late. I've been wall bumming for the last five minutes.
2) I was wall bumming for so long i marked the wall.