Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ĭn-effābĭlis, e, adj., unutterable, unpronounceable (Plinian): nomina, Plin. 5 praef. § 1: verba, id. 28, 2, 4, § 20.
Adv.: ĭneffābĭlĭter, unutterably, ineffably (late Lat.): Deus ineffabiliter artifex, Aug. Enchir. 89.