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sackcloth

cloth made of black goats' hair, coarse, rough, and thick, used for sacks, and also worn by mourners (Genesis 37:34; 42:25; 2 Samuel 3:31; Esther 4:1-2; Psalms 30:11, etc.), and as a sign of repentance (Matthew 11:21)

It was put upon animals by the people of Nineveh (Jonah 3:8).