King James Bible Dictionary

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).

A symbol of mourning
1 Kings 20:31-32; Job 16:15; Isaiah 15:3; Jeremiah 4:8; Jeremiah 6:26; Jeremiah 49:3; Lamentations 2:10; Ezekiel 7:18; Daniel 9:3; Joel 1:8

Worn by Jacob when it was reported to him that Joseph had been devoured by wild beasts
Genesis 37:34

Animals covered with, at time of national mourning
Jonah 3:8
Mourning

cloth used in making sacks or bags, a coarse fabric, of a dark color, made of goat's hair, (Isaiah 50:3; Revelation 6:12) end resembling the eilicium of the Romans. It, was used also for making the rough garments used by mourners, which were in extreme cases worn next the skin. (1 Kings 21:27; 2 Kings 6:30; Job 16:15; Isaiah 32:11)

SACK'CLOTH, noun [sack and cloth.] Cloth of which sacks are made; coarse cloth. This word is chiefly used in Scripture to denote a cloth or garment worn in mourning, distress or mortification.

Gird you with sackcloth and mourn before Abner. 2 Samuel 3:31.

Esther 4:1. Job 16:15.

SACK'CLOTHED, adjective Clothed in sackcloth.