knuckle

1. N-COUNT Your knuckles are the rounded pieces of bone that form lumps on your hands where your fingers join your hands, and where your fingers bend. 指關節

Brenda's knuckles were white as she gripped the arms of the chair. 佈倫達緊緊抓著椅子的兩個扶手時指關節白了。

2. rap sb's knuckles, rap sb on/over the knuckles→see rap