WordNet (r) 3.0 (2006):

knuckle down
    v 1: work very hard, like a slave [syn: slave, break one's
         back, buckle down, knuckle down]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "knuckle down":
   accede, accept, acquiesce, assent, be agreeable, buckle down,
   comply, consent, endeavor, face the music, go all out,
   go along with, hump, hump it, knock under, knuckle under, lay to,
   live with it, not resist, obey, ply the oar, relent, resign,
   spare no effort, submit, succumb, swallow it, swallow the pill,
   take, take it