In Sir Gawain and the Green Knight, the Green Knight claims that his wife was Sir Gawain's "fierce foe." In what ways might the lady be considered Sir Gawain's foe?
When it comes to the rules of knighthood, knights strictly follow them in order to maintain a respectable reputation among the people, other knights, and as representatives of the king for whom they serve. Examples of those knightly rules are explained in the story of Sir Gawain and the Green Knight as Gawain bows, kisses, and maintains his "courtesy," or manners. However, when Gawain's foe is a woman who does not have to follow the rules of knightly engagement, Gawain is at an inexperienced disadvantage. Gawain later compares himself to Biblical men like Adam, Samson, and David who were also weak in the presence of women, but who stood strong against traditional enemies of war. So, the Lord's wife was fierce because Gawain didn't have any training to combat "women's wiles."