knuckle down

1864 in American English, from the game of marbles.

• "You should knuckle down and do your homework!"
• "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."

• belly up to the bar
• get one's act together
• pull up one's socks
• roll up one's sleeves
• shape up