knuckle down
To get to work; to focus on a task.
You should knuckle down and do your homework!
ADJ
bony | bleeding, bloodied, bruised, grazed, swollen
VERB + KNUCKLE
crack
She cracked her knuckles nervously before sitting down to take the difficult exam.
bruise, graze, scrape, skin
KNUCKLE + NOUN
bone
PHRASES
rap sb on/over the knuckles
The teacher rapped the student on the knuckles for arriving late to class again.