Exactly What It Says on the Tin. A jock isn't always a jerk. Sometimes he is instead (or as well) Book Dumb (at best) or is Too Dumb to Live at worst. Much like his girlfriend, The Cheerleader of the Brainless Beauty variety, he's more concerned with his image then school work. If he's not a jerk it might be because he's too dumb to realize he's supposed to be (though the Jerk Jock can also be incredibly stupid like the caveman mentality of the Dumb Muscle). Often times the better he is at his respective sport, the dumber the jock is like a Genius Ditz and the more valued he is by his team and the facility who will make sure that he maintains his position regardless of his apparent mental incompetence.