Source : Webster's Revised Unabridged Dictionary (1913)

Knuckle \Knuc"kle\, v. i. [imp. & p. p. {Knuckled};; p. pr. &
   vb. n. {Knuckling}.]
   To yield; to submit; -- used with down, to, or under.

   {To knuckle to}.
   (a) To submit to in a contest; to yield to. [Colloq.] See {To
       knock under}, under {Knock}, v. i.
   (b) To apply one's self vigorously or earnestly to; as, to
       knuckle to work. [Colloq.]