Autopederasty refers to the incredibly rare ability for a man to penetrate his own anus with his penis. This practice is very uncommon since men do not have a penis long enough to reach their own anus.
An autopederast can insert his semi-erect or fully erect penis into his own anus. This technically is a form of masturbation, but it is incredibly uncommon. Due to the position and angle of the penis during the insertion, ejeculation is not considered physically possible.