Fear
The reason for the slight nick on Gawain's neck as a punishment for his fear is because in the story of Sir Gawain and the Green Knight, Gawain accepts a challenge from the Green Knight, who agrees to allow Gawain to strike him with his axe if the Green Knight can return the favor a year later. Gawain agrees to the challenge and cuts off the Green Knight's head, but the Green Knight picks up his head, mounts his horse, and rides away. A year later, the Green Knight returns and Gawain is fearful, but he keeps his promise and allows the Green Knight to strike him. The Green Knight gives Gawain a slight nick on the neck as a reminder of his fear.