knuckle down

1864 in American English, from the game of marbles.

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knuckle down (third-person singular simple present knuckles down, present participle knuckling down, simple past and past participle knuckled down)

1. (intransitive, idiomatic) To get to work; to focus on a task.

   You should knuckle down and do your homework!

   • 1939 November 10, “Following The War”, in The Chart‎, volume I, number 1, Joplin, Missouri: Joplin Junior College, page 4, column 1:

     As students all over the United States knuckle down to learning, the rumble of war drums once more proclaims Mars high man in Europe. Discarding morbid curiosity, every student should consider it vitally necessary to get a general picture of the causes, movements, and possible effects of World War II. The average U. S. citizen's knowledge of World War II will probably decide his role in it.

2. To submit; knuckle under.

• (get to work): belly up to the bar, get one's act together, pull up one's socks, roll up one's sleeves, shape up

• hunker down
• buckle down