International Standard Bible Encyclopedia (1915)
Small particles of earth. The word has several figurative and symbolic meanings:
(3) Throwing dust was an act expressive of execration. Thus, Shimei "cursed" David and "threw stones at him, and cast dust," literally, "dusted (him) with dust" (2Sa 16:13). So the crowd which Paul addressed at Jerusalem manifested their wrath against him by tossing about their garments and casting dust into the air (Ac 22:23).
(4) Shaking the dust off one’s feet against anyone (Mt 10:14; Mr 6:11; Lu 9:5; 10:11; Ac 13:51) is symbolic of renunciation, as we would say "washing one’s hands of him," an intimation that all further intercourse was at an end. It was practiced by the Pharisees on passing from Gentileto Jewish soil, it being a rabbinical doctrine that the dust of a heathen land defiles.
(5) It is also used figuratively for an innumerable multitude (e. g. Ge 13:16; 28:14; Job 27:16; Ps 78:27).
(6) The expression "Yahweh will make the rain of thy land powder and dust" (De 28:24) means the dust in consequence of the drought shall fall down instead of rain on the dry ground. In Judea and vicinity during a sirocco, the air becomes filled with sand and dust, which are blown down by the wind with great violence.
D. Miall Edwards