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]