1.C （指人）指关节　(with reference to persons) bone where a finger is joined to the hand or in a finger where it has a joint
2.C （指动物）膝关节，踝　(of animals) knee-joint, or the part joining the leg to the foot, especially as a joint of meat

　～ of pork　猪蹄

　The finger joints are called knuckles.　手指的关节叫做指关节。

 ♣ knuckle down to　(v.+adv.+prep.)
〈非正〉开始努力做… begin to work hard at sth
　He knuckled down to his research.　他开始致力于研究。
　I'm going to have to knuckle down to some serious study.　我得开始认认真真地学习了。
 ♣ knuckle under to　(v.+adv.+prep.)
〈非正〉向…认输；屈服于 accept sb else's authority
　He refused to knuckle under to any pressure.　他拒绝屈服于任何压力。