The Illustrated Bible Dictionary
Sackcloth
Easton’s Bible Dictionary (1897)
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; Psalm 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).