Merriam-Webster Collegiate Dictionary - gnaw

 verb  Etymology: Middle English ~en, from Old English gnagan; akin to Old High German gnagan to ~  Date: before 12th century  transitive verb  1.  a. to bite or chew on with the teeth; especially to wear away by persistent biting or nibbling a dog ~ing a bone  b. to make by ~ing rats ~ed a hole  2.  a. to be a source of vexation to ; plague anxiety always ~ing him  b. to affect like ~ing hunger ~ing her vitals  3. erode, corrode  intransitive verb  1. to bite or nibble persistently ~ing at his underlip  2. to produce an effect of or as if of ~ing waves ~ing away at the cliffs  • ~er noun