noun
a joint of a finger when the fist is closed
verb
to press or rub with the knuckles
adjective
relating to or resembling a knuckle
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She rapped her knuckle on the door before entering.
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I accidentally cracked my knuckle while playing basketball.
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I heard he got into a fight and ended up busting his knuckle.
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He was so nervous that he was biting his knuckles during the presentation.