"knuckle" in the noun sense
1. knuckle, knuckle joint, metacarpophalangeal joint
a joint of a finger when the fist is closed
"knuckle" in the verb sense
1. knuckle
press or rub with the knuckles
2. knuckle
shoot a marble while keeping one's knuckles on the ground
Source: WordNet® (An amazing lexical database of English)
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