Hastings' Dictionary of the Bible (1898)
SACK'CLOTH. This was a coarse Sitting in Sackcloth. fabric, as the Hebrew word "sack" means, made of black goats' hair and other materials, and worn either as a sign of repentance. Matt 11:21, or as a token of mourning. Gen 37:34; 2 Sam 3:31; Esth 4:1-2; Job 16:15; Ps 30:11; Isa 20:2; Rev 6:12. Hence the frequent occurrence in Scripture of figurative language connecting sackcloth with mourning and darkness. Isa 3:24; Eze 7:18; Eze 27:31; Am 8:10.