'Sackcloth' in the Bible

Then the people of Nineveh believed in God; and they called a fast and put on sackcloth from the greatest to the least of them.

When the word reached the king of Nineveh, he arose from his throne, laid aside his robe from him, covered himself with sackcloth and sat on the ashes.

But both man and beast must be covered with sackcloth; and let men call on God earnestly that each may turn from his wicked way and from the violence which is in his hands.