knuckle down

1864 in American English, from the game of marbles.

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

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

   You should knuckle down and do your homework!

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