Definition
Eisne (also spelled eigne, einsne, aisne, eign) is an archaic legal term meaning the senior or oldest son. It appears in historical English law primarily in the context of inheritance and land succession. The companion term eisnetia (also einetia) designates the share or portion of property belonging to the oldest son — the inheritance acquired by right of primogeniture, the rule under which the eldest male heir succeeded to the whole of an ancestor's real estate.
Common Confusion
The spelling variants of this term are numerous and inconsistent across historical sources — eigne, einsne, aisne, eign, eisne — and appear without standardization even within single treatises. Researchers encountering any of these forms in manuscript records, plea rolls, or early printed legal texts should treat them as equivalent. The term should not be confused with aigne or similar-sounding Norman French words that carried different legal freight in other contexts.
Why It Matters in Research
This is a term of pure historical interest. It has no operative role in modern law and appears almost exclusively in sources predating the nineteenth century, when primogeniture as a governing rule of inheritance was gradually displaced or abolished in most common law jurisdictions.
Researchers working in medieval and early modern English property law, feudal tenure, or descent of real property will encounter eisne and eisnetia in the context of disputes over inheritance rights, especially those involving questions of legitimacy (bastard eigne, mulier puisne) — a set of doctrines governing which son, legitimate or illegitimate, could claim the inheritance when birth order and legitimacy conflicted. In that context, eisne functions not merely as a label but as a legal status with procedural consequences.
The bastard eigne / mulier puisne doctrine is a closely related area where eisne appears with the most legal consequence. Under that doctrine, if an illegitimate eldest son (bastard eigne) had long possessed the estate and died seised of it, his possession could defeat the claim of a later-born legitimate son (mulier puisne) in certain circumstances. Researchers examining this doctrine will need to trace eisne through Bracton, Littleton, and Coke on Littleton, not through modern secondary literature, which rarely addresses it with the precision the original sources require.
The term is almost never indexed in modern legal databases. Corpus searches should include all spelling variants. In manuscript sources, the Norman French origin of the word means it may appear in contracted or anglicized forms that differ from any of the dictionary-attested spellings.
Historical Dictionary Support
Black's Law Dictionary (both editions) and Bouvier's are in full agreement on substance: eisne means the oldest son; eisnetia means his share acquired by primogeniture. All three sources cite Termes de la Ley and Kelham for eisne, and add Coke on Littleton and Cowell for eisnetia. The minor discrepancies across editions are typographical — artifacts of early printing — rather than substantive disagreements.
What the historical dictionaries do not do is situate eisne within the broader doctrine of bastard eigne and mulier puisne, which is where the term carried its most significant legal weight. For that doctrinal context, Coke on Littleton (referenced by all three sources at folio 166b) is the essential primary text, and Bracton's De Legibus et Consuetudinibus Angliae provides the earlier common law foundation. The dictionary entries are useful as identification aids but should not be mistaken for complete doctrinal accounts.
Jurisdictional Note
Eisne is a term of English common law and has no meaningful presence in American legal doctrine. American jurisdictions generally abolished primogeniture in the post-Revolutionary period, eliminating the operative context in which eisne and eisnetia functioned. The term appears in American legal dictionaries solely for historical and comparative reference.