Sackcloth

Sackcloth - (Easton's Bible Dictionary)

Cloth made of black goats' hair, coarse, rough, and thick, used for sacks, and also worn by mourners (Gen. 37:34; 42:25; 2 Sam. 3:31; Esther 4:1, 2; Ps. 30:11, etc.), and as a sign of repentance (Matt. 11:21). It was put upon animals by the people of Nineveh (Jonah 3:8).

Sackcloth - (Webster's 1828 Dictionary)

SACK'CLOTH, n. [sack and cloth.] Cloth of which sacks are made; coarse cloth. This word is chiefly used in Scripture to denote a cloth or garment worn in mourning, distress or mortification.

Gird you with sackcloth and mourn before Abner. 2 Sam 3.

Est 4. Job 46.