RKO are the initials for WWE wrestler Randy Keith Orton. It is also the name of his finishing move. DDP and Homicide have also used this move as one of their finishers, while Stone Cold Steve Austin uses a modified version as his finishing move, the Stone Cold Stunner.
RKOing people has become popular on Board 8 as a result of ExThaNemesis constantly performing the move on posters.
TimJab is truly the master of the RKO however. Anyone with a brain knows this.