a rocket that has been shot at an upward angle into another persons anus and becomes stuck.
When John was working at NASA they were testing miniature rockets and he was standing over one when it went off, after spending hours in the emergency room it was apparent that he had received a butt-rocket.
When the pressurized air that comes out of your tush, while sitting on the toilet, is so great in quantity and stature that it literally lifts you up off of the commode.