The Collaborative International Dictionary of English v.0.48:

Parnassus \Par*nas"sus\, n. [L., fr. Gr. ?.] (Anc. Geog. & Gr.
   Myth.)
   A mountain in Greece, sacred to Apollo and the Muses, and
   famous for a temple of Apollo and for the Castalian spring.
   [1913 Webster]

   Grass of Parnassus. (Bot.) See under Grass, and
      Parnassia.

   To climb Parnassus, to write poetry. [Colloq.]
      [1913 Webster]