sæcclāþ

Sackcloth comes from Old English sæcclāþ, a compound of sæcc (sack, from Latin saccus, from Greek sakkos, from Semitic — possibly Hebrew śaq) and clāþ (cloth). The word names a coarse fabric made from goat hair, camel hair, hemp, or flax. It was the cheapest, roughest textile available in the ancient Near East and Mediterranean. It was not clothing. It was the absence of clothing — what you wore when real clothing was too good for your circumstances.

In the Hebrew Bible, wearing sackcloth (Hebrew śaq) was a public sign of mourning, repentance, or extreme distress. Jacob wore sackcloth when he believed Joseph was dead (Genesis 37:34). The people of Nineveh wore sackcloth when Jonah prophesied their destruction (Jonah 3:5). Ashes were poured on the head to complete the gesture. Sackcloth and ashes meant: everything has been stripped away.

The King James Bible (1611) standardized 'sackcloth and ashes' as an English phrase. Shakespeare used it. It entered common idiom as a metaphor for exaggerated penitence or self-abasement. 'She was wearing sackcloth and ashes about the whole thing' means she was being ostentatiously sorry. The actual fabric has disappeared from daily life. The idiom has not.

The word sack itself — from the same Semitic root — names both the rough fabric and the bag made from it. A sack is a bag made from sack-cloth. The material and the object are the same word because the material had no other use worth naming. It was bag fabric. When it was worn on the body, it said: I am a bag.