can refer to a coarse cloth such as burlap but more commonly to a garment made from such material. Worn by penitents and ascetics, normally directly against their skin.
We are told the king issued orders for everybody, including men, women and children, and beasts, to stop eating and drinking, and to be covered with sackcloth. What sin can we suppose the beasts had committed that they must be doomed to starve, and be covered with sackcloth as an emblem of repentance?
Kersey and Lydia Graves. The Bible of Bibles (1879)
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I mean so to mortify myself, that instead of silks, I will wear sackcloth: for ouches and bracelets, leere and cadis
John Lyly. Euphues: The Anatomy of Wit (1578)
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“[...] a hermit’s got to sleep on the hardest place he can find, and put sackcloth and ashes on his head, and stand out in the rain, and—”
“What does he put sackcloth and ashes on his head for?” inquired Huck.
“I dono. But they’ve got to do it. Hermits always do. You’d have to do that if you was a hermit.”
Mark Twain. The Adventures of Tom Sawyer (1876)
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It happened I was with this Savka one fine May evening. I remember I was lying on a torn and dirty sackcloth cover close to the shanty from which came a heavy, fragrant scent of hay.
Anton Chekhov. The Witch and Other Stories.
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When my ablutions were completed, I was put into clean linen of the stiffest character, like a young penitent into sackcloth, and was trussed up in my tightest and fearfullest suit.