I gnashed my teeth
We have already witnessed this physical manifestation of deep psychological conflict in the cabin of Walton's ship, when Victor is first recovering from exposure to the arctic elements (I:L4:10). A grieving Victor gnashes his teeth at the mere thought of the Creature in II:1:6. When we next observe the phenomenon, it will be the Creature who gnashes his teeth in response to the frustration of all his hopes (III:3:13 and note). The prototype for this act is the Satan of Paradise Lost: compare VI.340.