Sackcloth - Vine's Expository Dictionary of New Testament Words
Sackcloth
[ 1,,G4526, sakkos ]a warm material woven from goat's or camel's hair," and hence of a dark color, Revelation 6:12; Jerome renders it saccus cilicinus (being made from the hair of the black goat of Cilicia; the Romans called it cilicium); cp. Isaiah 50:3; it was also used for saddle-cloths, Joshua 9:4; also for making sacks, e.g., Genesis 42:25, and for garments worn as expressing mourning or penitence, Matthew 11:21; Luke 10:13, or for purposes of prophetic testimony, Revelation 11:3.