Christianity

Catholic encyclopedia on "Hairshirt": https://www.newadvent.org/cathen/07113b.htm

A garment of rough cloth made from goats' hair and worn in the form of a shirt or as a girdle around the loins, by way of mortification and penance. The Latin name is said to be derived from Cilicia, where this cloth was made, but the thing itself was probably known and used long before this name was given to it. The sackcloth, for instance, so often mentioned in Holy Scripture as a symbol of mourning and penance, was probably the same thing; and the garment of camels' hair worn by St. John the Baptist was no doubt somewhat similar.