knuckle down
phrasal verb with knuckle verb uk/ˈnʌk.əl/ us/ˈnʌk.əl/
informal
开始努力工作;开始用功学习
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. 如果你想通过期末考试，就必须用功学习。