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]