Lewis & Short

Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.

Arganthōnĭus, ii, m., = Ἀργανθώνιος, a Tartessian king who lived to a great age, Cic. Sen. 19, 69; Val. Max. 8, 13, ext. 4 (cf. Herod. 1, 163); Plin. 7, 48, 49, § 154; Censor. 17.
Hence, Arganthōnĭăcus, a, um, adj., of or pertaining to Arganthonius, Sil. 3, 396.