The Collaborative International Dictionary of English v.0.48:

Metonic \Me*ton"ic\, a. [Cf. F. m['e]tonique.]
   Pertaining to, or discovered by, Meton, the Athenian.
   [1913 Webster]

   Metonic year or Metonic cycle. (Astron.) See under
      Cycle.
      [1913 Webster] Metonymic