To repent in sackcloth and ashes, or to lie down among ashes, was an external sign of self-affliction for sin, or of grief under misfortune. We find it adopted by Job, Job 2:8; by many Jews when in great fear, Esther 4:3; and by the king of Nineveh, Jonah 3:6 . The ashes of a red heifer were used in ceremonial purification, Numbers 19:1-22 .
Although written in 1859 by William Rand, this dictionary of the Bible from the American Tract Society is still relevant today.Wikipedia
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