means The knuckles are the rounded joints of your fingers where the bones meet, the bumps that stand up when you make a fist.

from From Middle English 'knokel,' a diminutive meaning 'little bone' or 'small joint,' related to the broader Germanic family of 'knock'-type words for small rounded lumps — you can hear its cousins in Dutch 'knokkel' and German 'Knöchel' (ankle/bone). Originally it could mean any joint, including the knee or knuckle of an animal's leg, before it settled onto the hand.