That of the citizens of Pydna, a city on the NE coast of Balkan Greece; Barrington Atlas map 50 grid C4. (Particularly famous as the site of the battle, in 168 BCE, which saw the Roman take-over of the Macedonian monarchy: see generally OCD4 s.v.)
For Pydna/Pydnaians in the Suda cf. tau 566.
In the present entry the name is in the accusative case and so evidently quoted from somewhere, e.g. Theopompus FGrH 115 F30a; alternatively, extracted from tau 566.