The Collaborative International Dictionary of English v.0.48:

Arcadian \Ar*ca"di*an\, Arcadic \Ar*ca"dic\, a. [L. Arcadius,
   Arcadicus, fr. Arcadia: cf. F. Arcadien, Arcadique.]
   1. Of or pertaining to Arcadia; pastoral; ideally rural; as,
      Arcadian simplicity or scenery.
      [1913 Webster]

   2. the dialect of Ancient Greek spoken by Arcadians.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

Arcadic
    n 1: the dialect of Ancient Greek spoken by Arcadians [syn:
         Arcadic, Arcadic dialect]