[1] (A marginal addition in ms. M.) The word occurs only in the nominative singular in Homer, where it appears related to the indeclinable adverb ἄγαν 'too much'. E. Risch takes it as a rare disyllabic root ending in a vowel, cf. the related verb ἄγαμαι 'admire' (Wortbildung der homerischen Sprache §3d, p.4). See H.-J. Mette's brief but useful entry in Lexikon des fruehgriechischen Epos I (fasc. 1, 1955) 62-63. It is, however, declined in later Greek as a first declension noun.
[2] Herodotus 6.61.1.
[3] Homer, Iliad 21.221. In its only other uses in Homer, Odyssey 16.243 and 3.227, it is used in the phrase ἄγη μ' ἔχει 'shock holds me'. It is necessary there to replace its synonyms θάμβος and σέβας to suit the metre. Mette translates it, "Gefuehl, dass der Gespraechspartner etwas Ausserordentliches, das Mass Ueberschreitendes tut oder sagt" ("the feeling that the other person in the conversation is doing or saying something extraordinary that oversteps the limits").
[4] The latter word is certainly related to the verb ἄγνυμι 'break', probably different from the root for amazement at that which breaks the limits, underlying the Headword.