In Ancient Greek, Persian courier was called ἄγγαρος (ángaros, “Persian mounted courier”), whence Latinangarius, which is probably from an East Asian language (e.g., Sanskritअजिरा (ajirā, “agile, swift”)). The word "angel" (ἄγγελος) is probably loaned from and related to Greek ἄγγαρος.
Nothing mortal travels so fast as these Persian messengers. The entire plan is a Persian invention; and this is the method of it. Along the whole line of road there are men (they say) stationed with horses, in number equal to the number of days which the journey takes, allowing a man and horse to each day; and these men will not be hindered from accomplishing at their best speed the distance which they have to go, either by snow, rain, heat, or by the darkness of night. The first rider delivers his despatch to the second and the second passes it to the third; and so it is borne from hand to hand along the whole line, like the light in the torch-race, which the Greeks celebrate to Vulcan. The Persians give the riding post in this manner, the name of Angarum.[a]
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Gascoigne, B. "Persian couriers". List of Subjects: P–R: Persia: History of Iran (Persia): Achaemenids: Imperial communication: 522-486 BC. Bamber Gascoigne; HistoryWorld. Archived from the original on 2015-05-12. Retrieved 2015-05-12.