To gnaw means to bite or chew on something repeatedly, usually something hard, wearing it down bit by bit. A dog gnaws on a bone, working at it patiently with its teeth. A mouse gnaws through wood to get into a pantry, its tiny teeth scraping away steadily until it creates a hole.

The word captures that persistent, determined quality of the action. You don't gnaw something in one quick bite. Instead, you keep at it: gnaw, gnaw, gnaw. Beavers gnaw through tree trunks to build their dams, their strong front teeth designed perfectly for the job.

Gnaw also describes a persistent, nagging feeling that won't go away. Worry might gnaw at you before a big presentation, or guilt might gnaw at your conscience after you've been unkind to a friend. Just like teeth working away at something hard, these feelings keep working away at your mind, refusing to leave you alone.