Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
Storms of sand and dust sometimes overtake Eastern travellers. They are very dreadful, many perishing under them. Jehovah threatens to bring on the land of Israel, as a punishment for forsaking him, a rain of "powder and dust" (Dt. 28:24).
To cast dust on the head was a sign of mourning (Josh. 7:6); and to sit in dust, of extreme affliction (Isa. 47:1). "Dust" is used to denote the grave (Job 7:21). "To shake off the dust from one's feet" against another is to renounce all future intercourse with him (Mt. 10:14; Acts 13:51). To "lick the dust" is a sign of abject submission (Ps. 72:9); and to throw dust at one is a sign of abhorrence (2 Sam. 16:13; comp. Acts 22:23).