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Related to knuckle down: out of whack, buckle down
To begin to work on something in a determined matter. If you want to get a passing grade this semester, you'll really need to knuckle down and study hard.
knuckle down (to something)
Fig. to get busy doing something. I want you to knuckle down to your work and stop worrying about the past. Come on. Knuckle down. Get busy.
1. Apply oneself seriously to some task or goal, as in The professor insisted that we knuckle down and get our papers in by Friday. Both this term and the rhyming synonym buckle down date from the 1860s, but the precise allusion in either is unclear.
2. See knuckle under.
To apply oneself earnestly to a task: We've been relaxing too long—it's time for us to knuckle down and finish this work.
Apply yourself to the job at hand. The phrase comes from the game of marbles, one of the once-popular children's street games. Players shot their “shooter” marble by clenching the marble in a fist with knuckles touching the ground, then launching it with a flick of the thumb. When it was a player's turn and his attention was elsewhere, he was reminded, “Okay, knuckle down.” A similar phrase, “buckle down” most likely came from the idea of tightening your belt before performing an arduous task.