knuckle downvi phrasalphrasal verb, intransitive: Verb with adverb(s) or preposition(s), having special meaning and not taking direct object--for example, "make up" [=reconcile]: "After they fought, they made up."
slang (apply yourself to a task)
努力 nǔ lì
You had better start knuckling down in school, or you'll never graduate.
knuckle down to [sth]vi phrasal + prep
slang (apply yourself)
埋头工作 mái tóu gōng zuò
埋头苦干 mái tóu kǔ gàn
We don't have much time for this project. We'd better knuckle down to work.