Sackcloth (Hebrew: שַׂק śaq) is a coarsely woven fabric, usually made of goat's hair. The term in English often connotes the biblical usage, where the Hastings' Dictionary of the Bible remarks that haircloth would be a more appropriate rendering of the Hebrew meaning.
In some Christian traditions (notably Catholicism), the wearing of hairshirts continues as a self-imposed means of mortifying the flesh that is often practiced during the Christian penitential season of Lent, especially on Ash Wednesday, Good Friday, and other Fridays of the Lenten season.