'Sackcloth' in the Bible

"Look, now," his advisors suggested, "we've heard that the Israeli kings are merciful. So let's clothe ourselves with sackcloth, tie our hair back with ropes, and go out to the king of Israel. Maybe he'll spare your life."

So they put on some sackcloth, tied their hair back with ropes, and approached the king of Israel. "Your servant Ben-hadad says this," they said. "Please let me live." "Is he still alive?" Ahab asked. "He's my brother."

Nevertheless, as soon as Ahab heard this message, he tore his clothes, put on sackcloth, and fasted. He even slept in sackcloth and wandered around meekly.