'Sackcloth' in the Bible

“Woe (judgment is coming) to you, Chorazin! Woe to you, Bethsaida! For if the miracles done in you had been done in Tyre and Sidon [cities of the Gentiles], they would have repented long ago in sackcloth and ashes [their hearts would have been changed and they would have expressed sorrow for their sin and rebellion against God].