The Collaborative International Dictionary of English v.0.48:

Gnaw \Gnaw\, v. i.
   To use the teeth in biting; to bite with repeated effort, as
   in eating or removing with the teeth something hard,
   unwieldy, or unmanageable.
   [1913 Webster]

         I might well, like the spaniel, gnaw upon the chain
         that ties me.                            --Sir P.
                                                  Sidney.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Gnaw \Gnaw\ (n[add]), v. t. [imp. & p. p. Gnawed (n[add]d); p.
   pr. & vb. n. Gnawing.] [OE. gnawen, AS. gnagan; akin to D.
   knagen, OHG. gnagan, nagan, G. nagen, Icel. & Sw. gnaga, Dan.
   gnave, nage. Cf. Nag to tease.]
   1. To bite, as something hard or tough, which is not readily
      separated or crushed; to bite off little by little, with
      effort; to wear or eat away by scraping or continuous
      biting with the teeth; to nibble at.
      [1913 Webster]

            His bones clean picked; his very bones they gnaw.
                                                  --Dryden.
      [1913 Webster]

   2. To bite in agony or rage.
      [1913 Webster]

            They gnawed their tongues for pain.   --Rev. xvi.
                                                  10.
      [1913 Webster]

   3. To corrode; to fret away; to waste.
      [1913 Webster]

   4. To trouble in a constant manner; to plague; to worry; to
      vex; -- usually used with at; as, his mounting debts
      gnawed at him.
      [PJC]

WordNet (r) 3.0 (2006):

gnaw
    v 1: bite or chew on with the teeth; "gnaw an old cracker"
    2: become ground down or deteriorate; "Her confidence eroded"
       [syn: erode, gnaw, gnaw at, eat at, wear away]

Moby Thesaurus II by Grady Ward, 1.0:

113 Moby Thesaurus words for "gnaw":
   ablate, abrade, abrase, afflict, agonize, ail, annoy, badger, bark,
   bedevil, beleaguer, bite, bother, burn, canker, chafe, champ, chaw,
   chew, chew the cud, chew up, chomp, consume, convulse, corrode,
   crucify, crumble, cut, devour, distress, eat, eat away, eat into,
   erase, erode, excruciate, fester, file, fray, frazzle, fret, gall,
   give pain, gnash, gnaw away, grate, graze, grind, gripe, gum,
   harass, harrow, harry, haunt, hector, hurt, inflame, inflict pain,
   irk, irritate, kill by inches, lacerate, martyr, martyrize,
   masticate, mouth, mumble, munch, nag, nettle, nibble, nibble away,
   nip, oxidize, pain, peeve, pester, pierce, pinch, plague, prick,
   prolong the agony, put to torture, rack, rankle, rasp, raze, rub,
   rub away, rub off, rub out, ruminate, rust, scour, scrape, scrub,
   scuff, skin, stab, sting, tease, torment, torture, trouble, tweak,
   twist, vex, wear, wear away, wear down, worry, wound, wring