The hair shirt, also called a "celice," is a garment woven of rough goat hair or horse hair. According to the Catholic Encyclopedia, the celice could be a literal shirt or a loin cloth. Its purpose was to mortify the flesh. By creating physical discomfort (think itchiness and chafing - ouch!), it reminded the wearer to resist temptations of the flesh, and was a sign of the wearer's penitence. Holy men and women, members of the clergy, and lay people could own and wear hair shirts.
Years ago I attended a talk by a faculty member at my alma mater called "Holy Underwear." Among many fun facts the medievalist speaker shared was the notion that people who wanted to wear their hair shirts too frequently were considered freaks, and might be told to dial it back. You wore your hair shirt for an itchy hour, in other words, not 24 / 7.
|~ Hair shirt belonging to St. Louis, 13th century ~|
~ St. Louis wore this shirt every Friday during Lent ~