Webster's Revised Unabridged Dictionary · 1913

Sackcloth

Sackcloth (?; 115) , noun

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