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Webster 1913 Edition

Adj.

[L.

Castalius

]

Of or pertaining to Castalia, a mythical fountain of inspiration on Mt. Parnassus sacred to the Muses.

Milton.

Adj.

Pertaining to Castalia, a cool spring on Parnassus, sacred to the muses; as Castalian fount.

Castalian ‎(comparative more Castalian, superlative most Castalian)

Of or relating to Castalia, a mythical fountain of inspiration on Mount Parnassus, sacred to the Muses.
(Can we find and add a quotation of Milton to this entry?)