Other forms: knuckled down; knuckles down; knuckling down
When you knuckle down, you really focus and get to work. Are you serious about finishing that art project by tomorrow? Then you'll have to knuckle down for the rest of the day!
Most experts believe that this North American phrase, first used around 1860, comes from the old-fashioned game of marbles. A player shooting a marble would place a knuckle on the floor or ground to get in the proper position. Likewise, someone getting started on a serious task needs to knuckle down. English has many idiomatic phrases that also mean "get ready to work hard," from roll up your sleeves to pull up your socks.