Apply yourself to the job at hand. The phrase comes from the game of marbles, one of the once-popular children's street games. Players shot their “shooter” marble by clenching the marble in a fist with knuckles touching the ground, then launching it with a flick of the thumb. When it was a player's turn and his attention was elsewhere, he was reminded, “Okay, knuckle down.” A similar phrase, “buckle down” most likely came from the idea of tightening your belt before performing an arduous task.