'Sackcloth' in the Bible

The men of Nineveh believed in God. They proclaimed a fast and dressed in sackcloth—from the greatest of them to the least.

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

Furthermore, both man and beast must be covered with sackcloth, and everyone must call out earnestly to God. Each must turn from his evil ways and from the violence he is doing.