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
see also:
knuckled knuckling to knuckle to to knock under knock