Strong's #4526: sakkos (pronounced sak'-kos)
of Hebrew origin (8242); "sack"-cloth, i.e. mohair (the material or garments made of it, worn as a sign of grief):-- sackcloth.
Thayer's Greek Lexicon:
1) a sack
1a) a receptacle for holding or carrying various things, as money, food, etc.
1b) a course cloth, a dark course stuff made especially from the hair of animals
1c) a garment of the like material, and clinging to the person like a sack, which was wont to be worn (or drawn over the tunic instead of the cloak or mantle) by mourners, penitents, suppliants and also by those who like the Hebrew prophets, lead an austere life
Part of Speech: noun masculine
Relation: of Hebrew origin H8242
Citing in TDNT: 7:56, 995
This word is used 4 times:
Matthew 11:21: "they would have repented long ago in sackcloth and ashes."
Luke 10:13: "repented, sitting in sackcloth and ashes."
Revelation 6:12: "became black as sackcloth of hair, and the moon"
Revelation 11:3: "a thousand two hundred and threescore days, clothed in sackcloth."