Derivational etymology starting from a ghost word, *mairō being assumed as the basis of the reduplicated μαρμαίρω and taken as a derivation base. The etymology implies a formal change, that of the vowel. It is typical of the Greek way of considering words in that only the nominative is accounted for, and Philoxenus does not seem to have cared about the fact that all the other case forms have an [r]: he does not try to account for the stem μαρτυρ- (nor do others) but for one specific case form, the nominative, which happens to have no [r], therefore can be derived from the verbal adjective (also a ghost form). Then the other case forms are derived by adjunction of a consonant (see Etym. Gudianum, Parallels). From the semantic point of view, this etymology relies on the juridical meaning of μάρτυς, the witness who has seen and can testify (whereas the older meaning, found in Homer, is different, the μάρτυς is the warrant of an oath, meaning for which "to make shine" would not provide a suitable etymology).

Herodian's explanation for the [r] was different and he saw in it a dialectal feature: Τὰ εἰς υρ βαρύνεται οἷον μάρτυρ. γίνεται δὲ ἐκ τοῦ μάρτυς κατὰ τὴν Αἰολέων διάλεκτον· ἐκεῖνοι γὰρ τὸ ς εἰς ρ μεταβάλλουσι τὸ οὗτος οὗτορ λέγοντες καὶ τὸ ἵππος ἵππορ· οὕτως οὖν καὶ μάρτυς μάρτυρ, ἀφ’ ἧς ἐκπίπτει πληθυντικὸν οἱ μάρτυρες παρά τε κωμικοῖς καὶ Ἱππώνακτι. οἱ δέ γε μάρτυροι ἀπὸ τῆς ὁ μάρτυρος εὐθείας οἷον «Ζεὺς δ’ ἄμμ’ ἐπιμάρτυρος ἔστω» (De prosodia catholica, Lentz III/1, p. 47): "words in -ur are barytone, as martur. It comes from martus in the Aeolic dialect: because Aeolians turn the [s] into [r], for instance they say houtor for houtos "this" and hippor for hippos "horse". So, therefore, martus, martur, from which then is inflected the plural martures in comedy and in Hipponax. Others have marturoi (Nom.pl.) from the nominative marturos as ‘Zeus be witness (epimarturos)’ (Il. 7.76)"