(sometimes figurative) To support or shore up something.
Try using a phone book to prop up the table where the foot is missing.
To play rugby in the prop position
(usually with "up" - see prop up) To position the feet of (a person) while sitting, lying down, or reclining so that the knees are elevated at a higher level.
To manually start the engine of a propeller-driven aircraft with no electric starter by pulling vigorously on one of the propeller blades using the hands, so that the propeller can catch ignition.