SENTENCE USAGE: After doing less than expected on his first pop quiz, Zack’s father warned him, "Son, if you expect to get a scholarship to go to college, you better knuckle down or you’ll be stuck workin’ at the mill with me!"
What do knuckles have to do with making better grades?
"Knuckle down" means getting sincere about something; applying oneself seriously to a job.
The origin of this expression is not certain, but one of the popular theories alludes to the game of marbles as being responsible. The game of marbles has evidently been enjoyed by children and even adults for a very long time. In fact, archaeologists have found examples of marbles that apparently date back thousands of years from ancient Egypt.
How does one play with marbles? Well, it’s a simple game involving two things: aim and precision. The goal is to hit your opponent’s marble with your own from a set distance. This is done either by throwing or rolling. For instance, one might decide to roll the marble along the ground like a bowling ball, while others choose to flick it with their thumbs. Some might even make silly attempts and toss the marble between their legs. The style of throw is up to the player.
Anyways, when things got serious, a person would apparently want to knuckle down by literally placing his or her knuckles against the ground in a fist. This would help steady the aim and help the person to more effectively direct the shot. Thus, the "getting serious" tone the phrase carries today might possibly derive from the game of marbles.