Definition of knuckle verb from the Oxford Advanced Learner's Dictionary

 

knuckle

 verb
verb
BrE BrE//ˈnʌkl//
 
; NAmE NAmE//ˈnʌkl//
 
Verb Forms present simple I / you / we / they knuckle
BrE BrE//ˈnʌkl//
 
; NAmE NAmE//ˈnʌkl//
 
he / she / it knuckles
BrE BrE//ˈnʌklz//
 
; NAmE NAmE//ˈnʌklz//
 
past simple knuckled
BrE BrE//ˈnʌkld//
 
; NAmE NAmE//ˈnʌkld//
 
past participle knuckled
BrE BrE//ˈnʌkld//
 
; NAmE NAmE//ˈnʌkld//
 
-ing form knuckling
BrE BrE//ˈnʌklɪŋ//
 
; NAmE NAmE//ˈnʌklɪŋ//
 
 
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Word Origin Middle English knokel (originally denoting the rounded shape when a joint such as the elbow or knee is bent), from Middle Low German, Middle Dutch knökel, diminutive of knoke ‘bone’. In the mid 18th cent. the verb knuckle (down) expressed setting the knuckles down to throw the taw (large marble) in a game of marbles, hence the notion of applying oneself with concentration. Phrasal Verbsknuckle down (to something)knuckle under
See the Oxford Advanced American Dictionary entry: knuckle