sackcloth

n. coarse fabric used to make sacks; garment made from coarse fabric and worn as a sign of mourning or repentance

Burlap, also known as sack cloth or hessian cloth, is a dense woven fabric.Burlap is usually made of jute and allied vegetable fibers. In the Bible, it is described as made of goats' hair or camels' hair.
See more at Wikipedia.org...

Noun
1. a garment made of coarse sacking; formerly worn as an indication of remorse
(hypernym) garment
2. a coarse cloth resembling sacking
(hypernym) fabric, cloth, material, textile

(n.)
Linen or cotton cloth such as sacks are made of; coarse cloth; anciently, a cloth or garment worn in mourning, distress, mortification, or penitence.
  

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)