Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Parnāsus and -os, also Parnas-sus or -os, i, m., = Παρνασός, afterwards Παρνασσός, a high mountain in Phocis with two peaks, sacred to Apollo and the Muses, at whose foot was the city of Delphi and the Castalian spring, now range of Liakhoura, Mel. 2, 3, 4; Plin. 4, 3, 4, § 7: mons ibi verticibus petit arduus astra duobus, Nomine Parnasus, Ov. M. 1, 317: biceps, id. ib. 2, 221; Pers. prol. 2: uterque, Stat. Th. 7, 346: Parnasi deserta per ardua, Verg. G. 3, 291: Parnasus gemino petit aethera colle, Luc. 5, 72.
Hence,