'Sackcloth' in the Bible

And 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.

And the tidings reached the king of Nineveh, and he arose from his throne, and laid his robe from him, and covered him with sackcloth, and sat in ashes.

but let them be covered with sackcloth, both man and beast, and let them cry mightily unto God: yea, let them turn every one from his evil way, and from the violence that is in his hands.