Compositional etymology. From βαιός "little" which has the same meaning as ἠβαιόν (because it is etymologically the same word), a privative compound is created, *ἀβαιός, which is then turned into ἠβαιός by a change of the vowel, for which a parallel is provided. The comment underlines the fact that, whereas as a rule a pathos (formal change) does not change the meaning of the word, in this case it does (it does in the case of ἄπειρος, ἤπειρος, too). However, the meaning of the compound is itself awkward: if βαιός means "small", a privative compound should mean "large", whereas this *ἀβαιός is assumed to mean "little". The compound is glossed by "that has no walk", which implies that the privative compound is not a privative of βαιός, but of the etymon of βαιός, namely, βαίνω. In modern terms, we would describe it as a case in which composition applies before derivation (*ἀ-βῶ → ἀβαιός → ἠβαιός)