All your raiment, to wit, your spoil and prey. See Leviticus 8:15, Leviticus 14:49. All these things had contracted some ceremonial uncleanness, either from the dead bodies which wore them, or the tents or houses where they were, in which such dead bodies lay, or from the touch of the Israelitish soldiers, who were legally defiled by the slaughters they made.
Related Commentaries of Numbers 31:20
Numbers 31:20
20 And purify all your raiment, and all that is made of skins, and all work of goats' hair, and all things made of wood.