A form of threesome involving one male and two females. The male is in between the two females and wears a dildo on his back like a tail. As the male penetrates one female he is simultaneously pulling out of the other and vice versa. The act is named after the man who pioneered it, Gary Lee Ottmonster.
This is also known as the reverse DP.
Joe ottmonstered those two girls he met at the bar last night.