The Collaborative International Dictionary of English v.0.48:

Knuckle \Knuc"kle\, v. i. [imp. & p. p. Knuckled;; p. pr. &
   vb. n. Knuckling.]
   To yield; to submit; -- used with down, to, or under.
   [1913 Webster]

   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.]
       [1913 Webster]