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It's time to knuckle down - finish your break.

You're terrified that you might actually have to knuckle down and study.

You should knuckle down and do your homework!

You're going to have to really knuckle down (to your work) if you want to pass your final exams.

He managed to knuckle down to his lessons long enough to pass his examination.

knuckle down

I think we've got to knuckle down and work hard.