Knuckle down

Meaning to apply oneself studiously and conscientiously to the task at hand and dates in this sense from the early 19th century. It derives from the game of marbles during the 18th century where players put their knuckles on the ground next to their marble, thumb at the ready. Players would flick their target marbles from a hand position where the knuckle was either pointing down towards the ground or was on the ground itself. Knuckling down was serious business for marbles players.