Originating from a bully prevention TV ad. Where the bully yells, "You suck McMiller!"
Used as an Insult.
Its been used in real life so many times as a joke, its become quite the popular saying, meaning that somone isnt very good at doing somthing.
Kid 1: *dies in a video game*
Kid 2: You suck McMiller!
Kid 1: *crys*
Kid 1: *trips over a rock*
Kid 2: Man, your really Sucking McMiller today