[1] : The Collaborative International Dictionary of English v.0.48

sackcloth \sack"cloth`\ ?; 115, n.
   linen or cotton cloth such as sacks are made of; coarse
   cloth; anciently, a cloth or garment worn in mourning,
   distress, mortification, or penitence.
   1913 webster

         gird you with sackcloth, and mourn before abner. --2
                                                  sam. iii. 31.
   1913 webster

         thus with sackcloth i invest my woe.     --sandys.
   1913 webster

[2] : WordNet (r) 2.0

sackcloth
     n 1: a garment made of coarse sacking; formerly worn as an
          indication of remorse
     2: a coarse cloth resembling sacking

[3] : Easton's 1897 Bible Dictionary

sackcloth
   cloth made of black goats' hair, coarse, rough, and thick, used
   for sacks, and also worn by mourners gen. 37:34; 42:25; 2 sam.
   3:31; esther 4:1, 2; ps. 30:11, etc., and as a sign of
   repentance matt. 11:21. it was put upon animals by the people
   of nineveh jonah 3:8.
   

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Sackcloth, definition of term: Sackcloth
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