Definition
A Latin term from Roman law meaning that which is against right or divine law — a wicked or impious thing or act. Nefas denotes conduct that violates a higher moral or religious order, as distinguished from mere violations of positive human law. In Roman legal and philosophical thought, nefas occupied the space of the gravely wrong, the sacrilegious, or the morally transgressive — acts that offended not merely civil authority but the divine or natural order underlying it.
The term functions as the antonym of fas, which denoted that which is permitted, right, or in accordance with divine law. Together, fas and nefas formed a conceptual pairing in Roman jurisprudence that distinguished the divinely sanctioned from the divinely forbidden, providing a moral framework beneath the structure of formal legal rules.
Common Confusion
Nefas is sometimes loosely equated with malum in se — that which is wrong in itself — and the concepts share family resemblance. But they are not synonymous. Malum in se is a common law concept focused on inherent moral wrongness as a matter of legal categorization; nefas is a Roman law concept rooted specifically in the violation of divine or sacred law. A researcher encountering nefas in a text should not automatically read it as a synonym for malum in se without attending to the Roman law context the author is working in.
Nefas should also not be confused with nefarious in its modern sense, though the words share a root. In legal sources, nefas is a technical term of art; nefarious is ordinary English with a general meaning of wicked or criminal.
Why It Matters in Research
Nefas appears almost exclusively in sources engaging with Roman law, civil law tradition, or natural law theory. Researchers encountering it in English-language materials are almost certainly reading a text with civilian or scholastic roots — a treatise on natural law, a work drawing on Roman jurisprudence, or a commentary in the civil law tradition. It is not a term that migrated into common law practice.
The primary navigational importance of nefas is its pairing with fas. Neither term fully explains itself without the other. Any corpus search or index lookup for nefas should be accompanied by a parallel search for fas. Burrill's explicitly flags this relationship; Black's does not.
Researchers working in natural law theory, early modern jurisprudence, or canonical and ecclesiastical law sources are most likely to encounter nefas in substantive use. In those contexts, the term carries real conceptual weight — marking the boundary between the humanly prohibited and the divinely condemned.
Because the term appears in historical dictionaries but not in contemporary legal usage, it functions in modern research primarily as a key to decoding older sources, not as operative legal vocabulary. If it appears in a document without the fas/nefas pairing, that is itself a signal worth noting.
Historical Dictionary Support
All four source dictionaries converge on a nearly identical definition: that which is against right or divine law, a wicked or impious act. Black's (both editions) and Bouvier's align word for word. Burrill's adds the most useful structural information by explicitly cross-referencing fas and noting the oppositional relationship between the two terms — making Burrill's the most instructive entry for a researcher trying to understand the term's function rather than merely its surface meaning.
All four sources cite Calvin — a reference to Johannes Calvinus's Lexicon Juridicum Juris Caesarei Simul et Canonici (1600), a standard reference work in the civil law tradition. This citation, consistent across all entries, confirms the term's grounding in the civilian scholarly tradition rather than English common law practice.
No source provides substantive elaboration on how nefas functioned in Roman legal proceedings or how it related to specific categories of Roman law. Researchers needing that depth should look beyond the historical dictionary shelf to primary civil law sources and natural law treatises.