Get down to work and apply oneself earnestly to it.
The BBC broadcast an antiques programme recently on the subject of marble collecting. The game was demonstrated and the presenters showed how a marble, which aficionados call a taw, was held by the crooked index finger and flicked by the thumb. They declared that "this is where the phrase 'knuckle down' came from". The same series had recently included the confidently expressed 'fact' that the origin of top dog was mediaeval saw pits (which is at best an uncertain derivation), so I had my doubts. A little research though has shown the knuckle down derivation to be correct.
In 1740, Thomas Dyche and William Pardon published A dictionary of all the words commonly us'd in the English tongue. In that they define the verb knuckle or knuckle down as:
"A particular phrase used by lads at a play called taw, wherein they frequently say, Knuckle down to your taw, or fit your hand exactly in the place where your marble lies."
As befits a work with such an ambitious title, they didn't stop at one definition and also included:
"Knuckle or knuckle down: to stoop, bend, yield, comply with, or submit to."
Neither of these meanings is now generally used. The former is still used in the game of marbles, where players still knuckle down. The latter has migrated to the more commonplace knuckle under. By 1864, Webster's Dictionary defined the knuckle down with the meaning we now use, i.e.:
"To apply oneself earnestly or vigorously."
The game of marbles can also claim the origin of another widely used term - for keeps.