Someone who checks out other men in the locker room while they are changing. "Knob" refers to a mans "rod" or "johnson", and "hawk" refers to the wandering eyes of the suspected knob watcher.
After third hour, little Billy sat and watched the other male students in his area change. Then one of the other boys noticed that Billy's eyes were wandering, so he started shouting "KNOB HAWK, WE GOT A KNOB HAWK, GET THE ROPE, GET THE ROPE!" And then little Billy was strung up and beaten like a toy pinata.