Definition from Wiktionary, the free dictionary
sackcloth (plural sackcloths)
- A coarse hessian style of cloth used to make sacks.
- (Usually paired with 'ashes'), garments worn as an act of penance. Now often used figuratively.
- After he realised the gravity of his crime he spent some time wearing sackcloth and ashes.
figuratively: garment worn as act of penance