'Sackcloth' in the Bible

The people of Nineveh believed God; and they proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them.

The news reached the king of Nineveh, and he arose from his throne, and took off his royal robe, covered himself with sackcloth, and sat in ashes.

but let them be covered with sackcloth, both man and animal, and let them cry mightily to God. Yes, let them turn everyone from his evil way, and from the violence that is in his hands.