a very coarse, rough fabric woven from flax or hemp.
sackcloth and ashes
used with allusion to the wearing of sackcloth and having ashes sprinkled on the head as a sign of penitence or mourning (Matt 11:21):they should, at least, be wearing sackcloth and ashes in token penance of the wrongs committed
sackcloth in other Oxford dictionaries
Definition of sackcloth in the US English dictionary