'Sackcloth' in the Bible

Then David said to Joab and to all the people with him, “Tear your clothes, put on sackcloth, and mourn before Abner.” And King David walked behind the bier.

Rizpah the daughter of Aiah took sackcloth and spread it out for herself on the rock, from the beginning of harvest [in the spring] until [the autumn] rain fell on them; and she allowed neither the birds of the sky to rest on their bodies by day, nor the beasts of the field [to feed on them] by night.