Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
serpentīnus, a, um, adj. [serpens], of or belonging to a serpent (eccl. Lat.): pectora, Ambros. in Luc. 2, 2, 51: serpentinos natos, Aug. Gen. contra Manich. 2, 26.