Sackcloth

Sackcloth (Hebrew שַׂק saḳ) is a term originally denoting a coarsely woven fabric, usually made of goat's hair. It later came to mean also a garment made from such cloth, which was chiefly worn as a token of mourning by the Israelites. It was furthermore a sign of submission (I Kings xx. 30 et seq.), and was occasionally worn by the Prophets. In Christianity, the tradition of wearing a sackcloth, also known as a hairshirt, continued as a self-imposed means of mortification of the flesh that is often worn during the Christian penetential season of Lent, especially on Ash Wednesday, Good Friday, and other Fridays of the Lenten season.

The Jewish Encyclopedia says the Old Testament gives no exact description of the garment, so its shape must be a matter of conjecture. According to Adolf Kamphausen, the saḳ was like a corn-bag with an opening for the head, and another for each arm, an opening being made in the garment from top to bottom. Karl Grüneisen ("Ahnenkultus," p. 80) thought the saḳ resembled the hairy mantle used by the Bedouins. Friedrich Schwally (in Stade's "Zeitschrift," xi. 174) concludes that it originally was simply the loin-cloth, which is an entirely different conception from that of Kamphausen or of Grüneisen. Schwally bases his opinion on the fact that the word "ḥagar" חָגַר (to gird) is used in describing the mode of putting on the garment (see Josh. i. 8; Isa. iii. 24, xv. 8, xxii. 12; Jer. vi. 26, xlix. 3). One fastens the saḳ around the hips ("sim be-motnayim," Gen. xxxvii. 34; "he'elah 'al motnayim," Amos viii. 10), while, in describing the doffing of the saḳ, the words "pitteaḥ me-'al motnayim" are used (Isa. xx. 2). According to I Kings xxi. 37 and II Kings vi. 30, it was worn next the skin.

...
Wikipedia