Slave's Leader: King! Let my people go!
King: Still gnawing away at that bone, are we? Carry on.
Slave's Leader: You cannot keep ignoring us.
Actually King along with his assistants are rowing on boat, Slave's Leader is on land telling him to give freedom to his people who is working under king as slaves.
After through research on word "Gnawing away" it shows "To bite or chew something a bit at a time". How does it suits here?