From nefās. Confer with nefārius, nefandus, infandus.
nefāstus (feminine nefāsta, neuter nefāstum); first/second declension
- (of a day) on which judgment could not be pronounced or assemblies of the people be held
- (figuratively) contrary to the sacred rites or to religion; irreligious, impious; wicked, profane, abandoned; unlucky, inauspicious; hurtful
- Ille et nefasto te posuit die... — "Whoever planted you that cursed day..." (Horace, Ode II.13)
- Quid intactum nefasti liquimus? — "What have we, in our impiety, left inviolate?" (Horace, Ode I.33)