The answer is:
These lines from "Sir Gawain and the Green Knight" refer to the nick on Gawain’s neck as a result of the Green Knight’s blow and to the green girdle that he wore as a reminder of his shame.
The Green Knight acknowledges his real name is Bertilak, the master of castle where Sir Gawain has just stayed. Because Sir Gawain fails to be completely honest, he guiltily wears the Lord's wife's girdle as a punishment for his lack of success.
