'Sackcloth' in the Bible

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

When word reached the king of Nineveh [of Jonah’s message from God], he rose from his throne, took off his robe, covered himself with sackcloth and sat in the dust [in repentance].

But both man and animal must be covered with sackcloth; and every one is to call on God earnestly and forcefully that each may turn from his wicked way and from the violence that is in his hands.