The definition of knuckle is a joint at the hand or toe that sticks out when you make a fist.(noun)
The joint in the middle of your finger that sticks forward when you make a fist is an example of the knuckle.
See knuckle in Webster's New World College Dictionary
Origin: ME knokyl < or akin to MDu & MLowG knokel, dim. of knoke, bone < IE *gneuĝ-: for IE base see knot
See knuckle in American Heritage Dictionary 4
Origin: Middle English knokel.
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