The headword is one of the standard terms for a Parthian (on which see generally OCD4 s.v. 'Parthia, Parthian empire'). See e.g. Stephanus of Byzantium s.v. Parthuaioi: 'a people once Skythian, later exiled or migrating to Media [...] They are called Parthoi [as pi 671] and Parthioi and Parthuaioi, and the territory Parthiaia and Parthuene [see pi 673: classified as a city!], and Parthuenos.' For the location, spanning present-day Iran and Turkmenistan, see Barrington Atlas map 97.
But even with that clear, the entry is problematic on two levels: the ambiguity indicated in the translation above (in? called?), and the introduction of Paphlagonia (cf. pi 826) into the picture. Bernhardy's solution to both difficulties was characteristically ingenious: emend the item to conform with Hesychius - *parqe/nios, potamo\s *paflagoni/as ('Parthenios, a river of Paphlagonia'; see Barrington Atlas map 86 grid C2 for this and one of the towns along it, Parthenia); one would need though, to accept that the entry was alphabetically misplaced.