A penitential garment woven from the hair of mountain goats or of camels, called in Latin a cilicium because the cloth from which it was made originated in Cilicia, where mountain goats abound.
Cloth woven of animal hair was in common use in the Near East from pre-Christian times. It was used for sacking, tents, and bad-weather clothing because of its impermeable quality. Worn next to the skin it becomes a true mortification and was used as such from early Christian times. It became the proper garb for public penitents, and ascetics seeking works of supererogation adopted its use for clothing and bedding.
Ancient and medieval rules of religious communities are silent on the use of the hair shirt. St. jerome spoke of it as the distinctive sign of the monk in the East. But, in the West, cassian denounced it as a form of monastic exhibitionism, a parading of virtue. However, it appears in the lives of many devout souls, in and out of the cloister. St. germain slept in a hair shirt and on a pile of ashes. St. Thomas becket was found, at his death, to be wearing a hair shirt that covered most of his body. St. louis of France wore one under his regal robes. In the late Middle Ages, the wearing of a hair shirt became standard practice for lent and advent, but it was reduced in size and form to a narrow strip of hair cloth worn around the waist or as a scapular.
Bibliography: j. van dodewaard, Lexikon für Theologie und Kirche, j. hofer and k. rahner, eds. (Freiburg 1957–65) 2:120–304. h. leclercq, Dictionnaire d'archéologie chrétienne et de liturgie (Paris 1907–53) 1:1623–25.
"Hair Shirt." New Catholic Encyclopedia. . Encyclopedia.com. (September 19, 2018). http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/hair-shirt
"Hair Shirt." New Catholic Encyclopedia. . Retrieved September 19, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/hair-shirt