The story
From the Bible Book of Esther 4:1 "When Mordecai learned of all that had been done, he tore his clothes, put on sackcloth and ashes, and went out into the city, wailing loudly and bitterly".
Etymology from Wiktionary, CC BY-SA 4.0.
wear sackcloth and ashes
Today it means: To publicly express regret for something done wrong. Full definition →