extremely wicked or villainous; iniquitous: a nefarious plot.
Origin: 1595–1605; < Latinnefārius wicked, vile, equivalent to nefās offense against divine or moral law (ne- negative prefix + fās law, right) + -ius-ious, with intervocalic s > r
1604, from L. nefarius "wicked, abominable," from nefas "crime, wrong, impiety," from ne- "not" (see un-) + fas "right, lawful, divinely spoken," related to fari "to speak" (see fame).