Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Perseus.
nĕfandus, a, um, adj. [ne-fari, lit. not to be mentioned, unmentionable; hence], impious, heinous, execrable, abominable (mostly post-Aug.; syn. infandus): sperate deos memores fandi atque nefandi, i. e. wrong, impiety, Verg. A. 1, 543: nefandum adulterium, Cic. Fragm. ap. Quint. 5, 11, 12 (al. nefarium): nefandum vehiculum, Liv. 1, 59: nefandissima quaeque tyrannicae crudelitatis exercuit, Just. 16, 4, 11: fraus, Juv. 13, 174: sacri, id. 15, 116.
Of persons: homo nefandus, Plin. 28, 1, 2, § 9: nefandi homines, Quint. 1, 3, 17.
Sup.: aususne es, nefandissimum caput? etc., Just. 18, 7, 10.
Hence, adv.: nĕfandē, impiously: multa nefande ausi, Sall. Fragm. ap. Prisc. p. 993 P. (dub: Dietsch. H. 1, 62, nefanda).
Sup.: nefandissime, Cassiod. Hist. Eccl. 10, 28.