The cowl (from the Latin, cuculla meaning "hood and rope") is a hood worn by members of religious orders. It also refers to a long, hooded cloak, with wide sleeves, worn by some Catholic and Orthodox monks when participating in the liturgy. Developed in the Middle Ages, they became the formal garment for those in monastic life. They were worn to give warmth to people who often spent long hours in unheated and drafty churches. . .
They are most commonly bestowed upon the monk at the time of his making solemn, or lifetime, vows. They are generally worn in conformity with the color of the monk's tunic, with the Benedictines wearing black, and other groups which follow the Rule of St. Benedict, e.g., the Camaldolese and Cistercians, wearing some form of white. Carmelites wear a white cowl, although their tunic and scapular are brown. Dominicans also wear black cowls, although over a white tunic and scapular. The cloak, with a hood, is also worn by nuns, in the same manner.
Some orders which are not part of the Benedictine tradition do not make use of this cloak. However, the Franciscans, Carthusians and Dominicans all wear cowls.
Among the Eastern Christians (Eastern Orthodox and Byzantine Catholics) the cowl developed into the koukoulion worn by monks of the Great Schema, the highest degree of monasticism in the Eastern Church. Currently the koukoulion is of two types: one is similar to the hood still worn by some Western monastic orders, the other takes the form of a stiff rounded hat (like a bowler hat without a rim) to which is attached an epanok. -- Wikipedia.
Go figure that one out little Jimmy.