Sitting down in, or covering one's self with, is the symbol of mourning (Job.2:8;42:6; Esther.4:1; Isaiah.61:3; Matthew.11:21).
To eat asides expresses figuratively mourning is one's food, i.e. one's perpetual portion (Psalm.102:9). "He feedeth on ashes," i.e., tries to feed his soul with what is at once humiliating and unsatisfying, on an idol which ought to have been reduced to ashes, like the rest of the tree of which it is made (Isaiah.44:20).
The ashes of a red heifer burnt entire (Numbers.19), when sprinkled upon, purified ceremonially the unclean (Hebrews.9:13) but defiled the clean person.
Catholic Diocese of Hong Kong:A Catholic Should Know: