Source : Webster's Revised Unabridged Dictionary (1913)

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.

         Gird you with sackcloth, and mourn before Abner. --2
                                                  Sam. iii. 31.

         Thus with sackcloth I invest my woe.     --Sandys.

Source : WordNet®

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