Lewis & Short

nĕ-fas, n. indecl., something contrary to divine law, sinful, unlawful, execrable, abominable, criminal; an impious or wicked deed, a sin, a crime (cf.: scelus, flagitium, peccatum).

  1. I. Lit.: quicquid non licet, nefas putare debemus, Cic. Par. 3, 2. 25; cf.: officia tua mihi nefas est oblivisci, id. Fam. 15, 21, 5: Mercurius, quem Aegyptii nefas habent nominare, id. N. D. 3, 22, 56: nefas est dictu, miseram fuisse talem senectutem, id. Sen. 5, 13: eum, cui nihil umquam nefas fuit, id. Mil. 27, 73: quibus nefas est … deserere patronos, Caes. B. G. 7, 40: corpora viva nefas Stygiā vectare carinā, Verg. A. 6, 391: fas atque nefas, right and wrong, id. G. 1, 505; Hor. Epod. 5, 87; cf. id. C. 1, 18, 10; Ov. M. 6, 585: per omne fas ac nefas, in every way, Liv. 6, 14, 10: nefas triste piare, Verg. A. 2, 184: illa dolos dirumque nefas in pectore versat, Certa mori, id. ib. 4, 563: lex maculosum edomuit nefas, i. e. adultery, Hor. C. 4, 5, 22: in omne nefas se parare, Ov. M. 6, 613: summum crede nefas animam praeferre pudori, Juv. 8, 83: belli, civil war, Luc. 2, 507; cf.: fugiens civile nefas, id. 7, 432: magnum nefas contrahere, Just. 24, 3: facere nefas, Vulg. Deut. 22, 21: operari, ib. Lev. 20, 13.
    Poet., of a wicked person, a wretch, monster: exstinxisse nefas tamen … Laudabor (i. e. Helen, as the destroyer of Troy), Verg. A. 2, 585.
    Also inserted as an interjection, O horrid! shocking! dreadful! quātenus, heu nefas! virtutem incolumem odimus, Hor. C. 3, 24, 30; cf.: heu nefas, heu! id. ib. 4, 6, 17: quosne, nefas! omnes infandā in morte reliqui? Verg. A. 10, 673: sequiturque, nefas! Aegyptia conjux, id. ib. 8, 688: Lavinia virgo Visa, nefas! longis comprendere crinibus ignem, O horrible! id. ib. 7, 73.
    Esp.: est nefas, it is forbidden, contrary to law, Varr. L. L. 6, 4.
  2. II. Poet., transf.
    1. A. A horrible or monstrous thing: Eumenides Stygiumque nefas, Luc. 6, 695; 1, 626: infernum, id. 7, 170; Stat. Th. 6, 942.
    2. B. Impossible: levius fit patientiā Quicquid corrigere est nefas, an impossibility (= ἀθέμιτον, ἀδύνατον), Hor. C. 1, 24, 20.