Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ārădĭus, a, um, adj., of or belonging to Aradus (now Ruad), a city on an island of the same name on the coast of Phœnicia (Plin. 5, 20, 17, § 78; Mel. 2, 7; cf. Mann. Phoenic. p. 309): quod genus endo marist Aradī fons, Lucr. 6, 891.
Hence, Ārădĭi, ōrum, m., the inhabitants of Aradus, Plin. 5, 20, 17, § 78.