 To knuckle down means to diligently apply oneself, and that expression originated with the game of marbles. A marble, also known as a tor, is held between a crooked index finger and flicked by the thumb. It is an essential rule of the game that the knuckle of the index finger must be placed down on the ground before taking a shot. The knuckle must also be placed in the exact position that the player's previous marble ended. A player breaking these rules will be quickly told to concentrate and knuckle down.