Boyd’s Bible Dictionary:

(coarse cloth). A coarse, goat-hair cloth used for making sacks and rough garments. The latter were worn next the skin by mourners and repentants (Gen. 37:34; 42:2534And Jacob rent his clothes, and put sackcloth upon his loins, and mourned for his son many days. (Genesis 37:34)
25Then Joseph commanded to fill their sacks with corn, and to restore every man's money into his sack, and to give them provision for the way: and thus did he unto them. (Genesis 42:25)
; 2 Sam. 3:3131And David said to Joab, and to all the people that were with him, Rend your clothes, and gird you with sackcloth, and mourn before Abner. And king David himself followed the bier. (2 Samuel 3:31); 1 Kings 21:2727And it came to pass, when Ahab heard those words, that he rent his clothes, and put sackcloth upon his flesh, and fasted, and lay in sackcloth, and went softly. (1 Kings 21:27); 2 Kings 6:3030And it came to pass, when the king heard the words of the woman, that he rent his clothes; and he passed by upon the wall, and the people looked, and, behold, he had sackcloth within upon his flesh. (2 Kings 6:30); Esther 4:1-21When Mordecai perceived all that was done, Mordecai rent his clothes, and put on sackcloth with ashes, and went out into the midst of the city, and cried with a loud and a bitter cry; 2And came even before the king's gate: for none might enter into the king's gate clothed with sackcloth. (Esther 4:1‑2); Job 16:1515I have sewed sackcloth upon my skin, and defiled my horn in the dust. (Job 16:15); Isa. 50:33I clothe the heavens with blackness, and I make sackcloth their covering. (Isaiah 50:3); Rev. 6:1212And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood; (Revelation 6:12)).

Concise Bible Dictionary:

Independence Arch and Peking Pass, Seoul, Korea—mourner in sackcloth in foreground.

Strong’s Dictionary of Greek Words:

of Hebrew origin (8242); "sack"-cloth, i.e. mohair (the material or garments made of it, worn as a sign of grief)
KJV Usage: