Sackcloth \Sack"cloth`\ (?; 115), 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.

Gird you with sackcloth, and mourn before Abner. --2 Sam. iii. 31.

Thus with sackcloth I invest my woe. --Sandys.

Source: Webster's Revised Unabridged Dictionary (1913)