American Dictionary of the English Language

SACK'CLOTH, noun [sack and cloth.] Cloth of which sacks are made; coarse cloth. This word is chiefly used in Scripture to denote a cloth or garment worn in mourning, distress or mortification.

Gird you with sackcloth and mourn before Abner. 2 Samuel 3:31.

Esther 4:1. Job 16:15.