The Septuagint always translated Malach by Ἄγγελος, even in that first sentence of the prophet, which in our version bears his name.  Angelos had simply meant a messenger in Greek, as it still does; but it acquired the especial signification of a heavenly messenger, both in its own tongue, and in the Latin, whither Angelus was transplanted with this and no other sense. (History of Christian Names, Yonge, 1884)