'Sackcloth' in the Bible

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

When the news reached the king of Nineveh, he got up from his throne, took off his royal robe, put on sackcloth, and sat on ashes.

Every person and animal must put on sackcloth and must cry earnestly to God, and everyone must turn from their evil way of living and from the violence that they do.