The headword (nom./acc. singular) is attested in this form only in lexicography. Euripides uses it numerous times in plural oblique forms. (e.g. Alcestis 873 (Web address 1)) Cf. alphaiota 4 for a related verb.
[1] The corresponding entry in pseudo-Zonaras (s.v.), which is clearly derived from the same source, applies this gloss differentially to a word similar (and, in late Greek, homophonous) to the headword, ἔαγμα .