The Collaborative International Dictionary of English v.0.48:

knuckle under \knuc`kle un"der\, v. i.
   To yield; to submit; -- usually used with to.
   [PJC]

WordNet (r) 3.0 (2006):

knuckle under
    v 1: consent reluctantly [syn: yield, give in, succumb,
         knuckle under, buckle under]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "knuckle under":
   accede, accept, acquiesce, assent, be agreeable, bow, capitulate,
   cave, comply, consent, defer, face the music, go along with,
   knock under, knuckle, knuckle down, live with it, not resist, obey,
   relent, resign, submit, succumb, swallow it, swallow the pill,
   take, take it