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.
His bones clean picked; his very bones they gnaw.
2. To bite in agony or rage.
They gnawed their tongues for pain. --Rev. xvi.
3. To corrode; to fret away; to waste.
4. To trouble in a constant manner; to plague; to worry; to
vex; -- usually used with at; as, his mounting debts
gnawed at him.
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