Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Perseus.
Pīrĭthŏus, i, m., = Πειρίθοος, son of Ixion, king of the Lapithœ, husband of Hippodamia, and friend of Theseus. After the death of Hippodamia, he descended, in company with Theseus, to the infernal regions, to carry away Proserpine; but was, together with Theseus, seized and detained in chains. Theseus was afterwards delivered by Hercules, who vainly endeavored to save Pirithoüs also, Ov. M. 8, 302 sq.: amatorem trecentae Pirithoum cohibent catenae, Hor. C. 3, 4, 79; 4, 7, 28: Aegidae Pirithoique fides, Ov. P. 2, 6, 26.