Nibble

verb

1. To bite or chew on with the teeth
2. To make by gnawing

   "rats gnawed a hole"

3. To be a source of vexation to plague

   "anxiety always gnawing him"

4. To affect like gnawing

   "hunger gnawing her vitals"

5. Erode, corrode
6. To bite or nibble persistently

   "gnawing at his underlip"

7. To produce an effect of or as if of gnawing

   "waves gnawing away at the cliffs"

phrasal verb

1. To be a source of worry or concern to (someone)

   "This problem has been gnawing at me day and night."