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]