NOUN (2)

1. a garment made of coarse sacking; formerly worn as an indication of remorse;

2. a coarse cloth resembling sacking;

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]

WordNet (r) 3.0 (2006):

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

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).