Source : Webster's Revised Unabridged Dictionary (1913)

Gnaw \Gnaw\, v. t. [imp. & p. p. {Gnawed}; 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.

            His bones clean picked; his very bones they gnaw.
                                                  --Dryden.

   2. To bite in agony or rage.

            They gnawed their tongues for pain.   --Rev. xvi.
                                                  10.

   3. To corrode; to fret away; to waste.

Gnaw \Gnaw\, v. i.
   To use the teeth in biting; to bite with repeated effort, as
   in eating or removing with the teethsomething hard, unwiedly,
   or unmanageable.

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

Source : WordNet®

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