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; 42:25; 2 Sam. 3:31; 1 Kings 21:27; 2 Kings 6:30; Esther 4:1-2; Job 16:15; Isa. 50:3; Rev. 6:12).
Concise Bible Dictionary:
A rough cloth made of hair, of which sacks and coarse clothing was made. When put on as a symbol of sorrow or repentance it was worn next the skin, and not taken off at night: it was often associated with ashes (1 Kings 21:27; 2 Kings 6:30; Job 16:15; Joel 1:13; Rev. 6:12).
Independence Arch and Peking Pass, Seoul, Korea—mourner in sackcloth in foreground.
Strong’s Dictionary of Hebrew Words:
Meaning:
from 8264; properly, a mesh (as allowing a liquid to run through), i.e. coarse loose cloth or sacking (used in mourning and for bagging); hence, a bag (for grain, etc.)
KJV Usage:
sack(-cloth, -clothes)