Cloth made of black goats' hair, coarse, rough, and thick, used for sacks, and also worn by mourners (Genesis 37:34; 42:25; 2Sam. 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).
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Sackcloth'". "Easton's Bible Dictionary".