The unglossed headword is otherwise unattested (apart from an unaccented appearance on papyrus in a magical incantation), but Adler's note is 'cf. Steph. Byz. v *da/ai'. In other words, this Suda entry may be connected with the Daai, glossed by Stephanus as a "Skythian people". They were called Dahae in Latin, also (A)parni, and were famous for their horse archers.
Barrington Atlas map 97 grid B1 (in present-day Turkmenistan).