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sackcloth

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).