Definition
Latin: "to defend with one hand." A formal denial of an accusation made upon oath. In early English legal procedure, this phrase described the act by which a defendant swore to the falsity of a plaintiff's claim — the foundational act in the now-abolished practice of wager of law. The defendant would deny the charge under oath, typically supported by a fixed number of compurgators (oath-helpers) who swore to the defendant's credibility. The phrase is sometimes used as a synonym for wager of law itself, though strictly speaking it describes the sworn denial that initiated that process.
Common Confusion
Defendere unica manu is occasionally treated as identical to wager of law, but the terms are not perfectly interchangeable. Wager of law (or law wager) is the broader procedural institution — the entire trial-by-oath mechanism. Defendere unica manu refers specifically to the defendant's sworn denial, the act that set the wager of law in motion. Think of the latter as one component within the former. Historical dictionaries tend to collapse the distinction; researchers encountering either term in old pleading records should look for both.
Why It Matters in Research
This term appears almost exclusively in pre-modern English legal sources — primarily medieval and early modern plea rolls, Year Books, and common law treatises predating the abolition of wager of law by statute in 1833 (3 & 4 Will. IV, c. 42). Researchers working in those materials need to recognize it as procedural language, not substantive doctrine.
Two traps are common. First, the Latin phrase will appear in pleading records without translation or gloss; a researcher unfamiliar with compurgation procedure may mistake it for a general denial or an unrelated oath form. Second, because wager of law was available only in specific actions — chiefly debt and detinue — encountering defendere unica manu in a source is itself a signal about the nature of the underlying claim. It narrows the procedural universe considerably and can help researchers identify the action type when the record is otherwise ambiguous.
The phrase has essentially no application in modern law. It survives in legal dictionaries for historical completeness and appears in secondary literature on medieval and early modern English procedure. Researchers tracing the history of oath-based proof, the decline of compurgation, or the development of trial by jury will find it a useful index term.
Within the Law Mind corpus, this term connects directly to materials on wager of law, compurgation, and the older forms of action in debt and detinue. It also provides context for understanding why assumpsit developed as a workaround — plaintiffs recast debt claims as assumpsit precisely to avoid giving defendants the option to wage their law.
Historical Dictionary Support
Rapalje & Lawrence and Black's second edition are in complete agreement, offering identical definitions: "to wage law; a denial of an accusation upon oath." Both cross-reference Blackstone's Commentaries (Book III, chapter 22) and Stephen's Commentaries, the standard secondary authorities on the subject.
Neither source elaborates on the distinction between the sworn denial itself and the broader wager of law procedure — a gap that reflects the dictionaries' summary character rather than any substantive disagreement. Blackstone's treatment in Book III remains the fullest accessible account of how the process actually worked in practice, including the role of compurgators and the ritual form of the oath. Stephen's Commentaries provide useful context on the procedure's decline. Researchers wanting more than a definitional gloss should go directly to those treatises rather than relying on the dictionary entries.
Jurisdictional Note
Wager of law, and with it defendere unica manu, was a creature of English common law and was formally abolished in England in 1833. American courts inherited common law pleading traditions but wager of law had fallen into disuse in most American jurisdictions well before independence; it was never a live procedural option in U.S. federal courts. The term has no operative legal significance in any current jurisdiction.