Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
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 (Mt. 11:21). It was put upon animals by the people of Nineveh (Jonah 3:8).