The Collaborative International Dictionary of English v.0.48:

Megalesian \Meg`a*le"sian\, a. [L. Megalesius, fr. Gr. Mega`lh
   the Great, a surname of Cybele, the Magna Mater.]
   Pertaining to, or in honor of, Cybele; as, the Megalesian
   games at Rome.
   [1913 Webster]