knuckle [ME] In medieval times a knuckle was the rounded shape made by a joint like the elbow or knee when bent, but over the years it became limited to the joints of the fingers. The word may ultimately be related to knee [OE]. Someone prepared to knuckle down to something is ready to concentrate on a task, but the phrase originally comes from a game. People playing marbles from the 18th century set their knuckles down on the ground before shooting or casting the ‘taw’, a large marble. Something which threatens to go beyond the limits of decency can be described as being near the knuckle [L19th]. This was originally used more generally to mean ‘close to the permitted limits of behaviour’.