Definition of knuckle verb from the Oxford Advanced American Dictionary

knuckle

verb
/ˈnʌkl/
 
 
PHRASAL VERBS

knuckle down (to something)

(informal) to begin to work hard at something synonym get down toI'm going to have to knuckle down to some serious study.knuckle downknuckle down to

knuckle under (to someone/something)

(informal) to accept someone else's authorityknuckle underknuckle under to