Jankyn, the Wife's fifth husband, is a total departure from her usual marital appetite because he is young, poor, and extremely difficult to dominate. Jankyn is a clerk, or educated person, and in keeping with the portrayal of clerks throughout the Prologue, he often spouts antifeminist statements. Jankyn takes so much delight in these sentiments that he reads from a book collection of them every night for his own enjoyment.
The Wife tells us that Jankyn was the husband she loved best, despite the fact that he beat her and, when they were first married, refused to bow to her authority. Much of her love for Jankyn seems to stem from his ability to satisfy her in bed.
Despite the Wife's claim to love Jankyn best, his character doesn't come off so well in the Prologue. He beats the Wife and seems to take pleasure in his misogyny. Yet, according to the Wife, Jankyn is the husband with whom she reaches the greatest harmony (or what passes for accord for her), gaining domination over his mind, body, and tongue after a cataclysmic physical fight.
The character of Jankyn may actually represent the clerkly class in general. So the Wife's eventual domination of him could represent her fantasy of definitively answering clerical antifeminism. Jankyn's presence also rounds out the Wife's character, revealing her to be as vulnerable to love as the next person.