The Greek word "ἔθνος" [eth(h)nos] means "nation" or "social group". Remembering that the Greek ending of the nominative case "-os" corresponds to the Georgian "-i", we have the right to write the word as "ethni" (ἔθνη). Moreover, that's the form in which the word occurs in the Bible, where it means "pagans".

The Kartvelian word "ethni" is the Old Kartvelian plural of the noun "ethi". Now it remains only to recall a number of Georgian provinces formed with the help of the morpheme "-ethi": Kakheti, Imereti, Svaneti, etc. We see that the morpheme "ethi" used to mean "people", "nation". Then it is a Kartvelian word, and not a Greek one. The direction of borrowing is not in doubt, since in the Greek language the word appeared in the plural form without any understanding that it was plural (“ethnos”).