1. （尤指前期不努力之后）开始努力工作（或学习）
    If someone knuckles down, they begin to work or study very hard, especially after a period when they have done very little work.

    e.g. The only thing to do was knuckle down and get on with some serious hard work...
           唯一能做的就是开始努力工作，好好干出点儿成绩来。
    e.g. He managed to knuckle down to his lessons long enough to pass his examination.
           他在离考试还有足够长时间的时候就开始认真学习，从而通过了考试。

1. work very hard, like a slave

    Synonym: slavebreak one''s backbuckle down