The headword -- an alternative spelling for fe/rtatos, -h, -on (bravest, best); cf. phi 232 -- is an irregular superlative, in the neuter nominative/vocative/accusative singular and masculine accusative singular, of the adjective a)gaqo/s, -h/, -o/n, good; see generally LSJ s.v. It is evidently extracted from somewhere; perhaps from this form's first attestation at Homer, Iliad 9.110 (web address 1), where Nestor endeavors to counsel Agamemnon about Achilles, a man most brave.
[1] The headword is identically glossed in the Synagoge (phi73), Photius' Lexicon (phi118 Theodoridis), Etymologicum Magnum 790.56 (Kallierges), and similarly elsewhere; see De Stefani, p. 66. See also e.g. Hesychius phi310 s.v. fe/riste and an instance (among others in Homer) of this form, the masculine vocative singular, at Homer, Iliad 6.123 (web address 2).