Marble players may be credited for this expression, which dates back to 17th century England. One rule in the game provides that a player must shoot directly from the spot where his or her marble lands; this requires that the player knuckle down, that is, put his knuckles on the ground. Since marbles was a popular pastime with adults as well as children at the time, knuckle down possibly came to express earnest application to any job. However, the bones of the spinal column were also known as “knuckles” in the 17th century, and the expression may derive from the act of “putting one’s back into a task.”