The Monk, Chaucer tells us, is a manly man. The Monk's favorite past-time is hunting, and to this end he keeps gorgeous (and probably expensive) horses and greyhounds. Like the Prioress, the Monk is all sorts of things that, as a religious figure, he should probably not be – a hunter, overfed, expensively-dressed in fur and gold jewelry, and a cultivator of expensive habits. But the Monk is willing to admit that he doesn't live a traditional religious life of hard work, study, and fasting, claiming as his excuse that he is a modern man, disdainful of the old traditions. So, out with the old fuddy-duddies like Augustine, who would have the monk slaving away over his books in a cloister, and in with the new – the new, in this case, being a comfortable life of sport, fine food and clothing, and amusements outside the monastery's walls.
Of the Monk's physical appearance, we learn that he is fat, bald, and greasy, with eyes that roll in his head. In medieval physiognomy, the practice of drawing conclusions about someone's character from their physical appearance, rolling eyes like this might be a sign of impatience and lust for food and women. This part of the Monk's portrait foreshadows the interaction between the Monk and the Host after the Tale of Melibee. At this point, before asking him to tell a tale, the Host praises the Monk's brawn and bulk and laments that he is a religious figure because, were the Monk not pledged to celibacy, he would surely impregnate lots of women! The Host says that he thinks the Monk would be a stud if given the opportunity, but considering the Monk's lack of respect for the "old" traditions of the religious life (and that mysterious love-knot pendant tying his hood), we think it's likely that he probably already is one.
With the Monk's portrait, we see another satire of religious figures who are supposed to live a monastic life of deprivation and hard work, but instead live a life of luxury and ease. Similar to the Prioress, the Monk is doing all kinds of things which, were he really pious, he would not. The Monk, though, is more self-aware about his departure from the pious life, taking the defensive stance of being a "modern" man, an excuse that rings somewhat hollow to discerning ears.