knuckle down

informal

to start working or studying hard:
You're going to have to really knuckle down (to your work) if you want to pass your final exams.
(Cambridge Advanced Learner's Dictionary & Thesaurus からの knuckle down の定義 © Cambridge University Press)

アメリカ英語辞典
to start working hard:
You’re going to have to knuckle down and catch up on your schoolwork.
(Cambridge Academic Content Dictionary からの knuckle down の定義 © Cambridge University Press)

- Cambridge Dictionary