'Sackcloth' in the Bible

David ordered Joab and all the people who were with him, "Tear your clothes, put on sackcloth, and mourn for Abner." King David walked behind the funeral procession,

Then Aiah's daughter Rizpah grabbed some sackcloth and spread it out for herself on the rock where her children had been hanged from the beginning of harvest until the first rain fell from the sky. She would not allow any scavenger birds to land on them during the day nor the beasts of the field to approach them at night.