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]

Moby Thesaurus II by Grady Ward, 1.0:

102 Moby Thesaurus words for "gnawing":
   ablative, abradant, abrasive, ache, aching, acute, acute pain,
   afflictive, agonizing, angina, atrocious, attritive, backache,
   bellyache, bite, biting, boring pain, cephalalgia, chafing,
   charley horse, colic, collywobbles, cramp, cramping, cramps, crick,
   cruel, darting pain, distressing, earache, erosive, excruciating,
   fret, fretting, fulgurant pain, galling, girdle pain, grave,
   grinding, gripe, gripes, griping, gut-ache, hard, harrowing, harsh,
   headache, heartburn, hemicrania, hitch, hurtful, hurting,
   jumping pain, kink, lancinating pain, megrim, migraine, nip,
   odontalgia, otalgia, painful, pang, paroxysm, paroxysmal, piercing,
   pinch, poignant, prick, pungent, pyrosis, racking, rasping,
   seizure, severe, sharp, sharp pain, shoot, shooting, shooting pain,
   sick headache, spasm, spasmatic, spasmic, spasmodic,
   splitting headache, stab, stabbing, stabbing pain, stinging,
   stitch, stomachache, thrill, throbbing pain, throes, toothache,
   tormen, tormenting, torturous, tweak, twinge, twitch, wrench