The headword is attested in this form (dative singular) in the Lexicon Gudianum and the derivative entry of ps.-Zonaras (799.27). Adler also cites the Ambrosian Lexicon (1465). The genitive form e)ph/tuos occurs in Homer, Odyssey 21.306 (web address 1), and all other attestations of all forms of the word seem to derive from commentary on that passage.
For apparently related words, see epsilon 2216, epsilon 2217.