Extended Strong's Greek Lexicon (Abbott-Smith based)
sackcloth
Usage: σάκκος also written σάκος, -ου, ὁ (cf. Heb. שַׂק, which it renders in LXX), __1. a coarse cloth, sackcloth, usually made of hair: Rev.6:12. __2. Anything made of sackcloth; __(a) a sack (Gen.42:25, al.); __(b) a garment of sackcloth, expressive of mourning or penitence: Mat.11:21, Luk.10:13, Rev.11:3.† (AS)