Definition
A Latin term from Roman civil law meaning to undertake an obligation on behalf of another person with the intent of becoming personally liable in that person's place. The act of expromittere extinguishes the original debtor's obligation and substitutes the undertaking party — the expromissor — as the new, sole obligor. This is a species of novation by change of debtor, accomplished not through the original debtor's initiative but through the voluntary assumption by a third party, accepted by the creditor.
Common Confusion
Expromittere is closely related to, but distinct from, general suretyship or guaranty. A surety or guarantor remains liable alongside the original debtor as a secondary obligor; the original debt persists. Expromittere goes further: the original debtor is released entirely, and the expromissor steps into that debtor's legal position as the primary and exclusive obligor. In modern terminology, expromittere corresponds most precisely to a novation by delegation or assumption of debt with release, not a simple guarantee. Researchers encountering this term in historical civil law sources should resist the instinct to map it onto common-law suretyship concepts, which carry different structural assumptions.
Core Elements
The act of expromittere required three operative conditions under Roman civil law:
1. A third party voluntarily undertakes to satisfy an existing obligation belonging to another.
2. The creditor accepts this substitution, which is essential — the creditor's consent distinguishes expromittere from mere voluntary assumption between debtor and third party.
3. The original debtor is discharged from the obligation, which distinguishes the act from accessory liability such as fideiussio (suretyship).
Why It Matters in Research
Expromittere appears almost exclusively in civil law and Roman law sources. Researchers working in the Law Mind corpus will encounter it in treatises on obligations, novation, and suretyship, particularly in works that draw heavily on Justinian's Corpus Juris Civilis. It is not a term that migrated into common law vocabulary in any meaningful way, so encountering it in an English or American legal source is a strong signal that the author is reasoning from civilian or mixed-jurisdiction principles.
The key research trap is conflating expromittere with fideiussio, the Roman analog to suretyship. Both involve a third party undertaking liability on behalf of a debtor, but fideiussio is accessory (the original debtor remains bound) while expromittere is substitutive (the original debtor is released). Historical sources do not always make this distinction explicit, and some older treatise writers use the terms loosely. When reading historical dictionary entries — including the sources behind this entry — note that the one-sentence formulation "undertake for another, with the view of becoming liable in his place" technically describes both arrangements unless the release of the original debtor is specified. Burrill's entry adds nothing beyond Black's, suggesting both drew from the same civilian lexicographer, Calvin's Lexicon Juridicum.
Researchers tracing novation doctrine across civil and common law traditions should use expromittere as a pivot term: it connects Roman novation theory, Scots law (which retained civilian concepts of delegation and novation), Louisiana law, and the broader civilian tradition. It will not appear in English equity or common law cases in its Latin form, but the underlying concept — discharge of original debtor by substitution — surfaces in cases involving assumption of debt and creditor consent.
Historical Dictionary Support
All three source dictionaries reproduce substantively identical definitions, each tracing the term to Calvin's Lexicon Juridicum (Calvinus), a standard reference for Roman law terminology in early modern legal scholarship. Black's (1st and 2nd editions) and Burrill are in complete agreement, which reflects not independent corroboration but a shared civilian source rather than independent analysis. None of the historical entries distinguishes expromittere from fideiussio, and none elaborates on the requirement of creditor consent — a gap that matters for researchers trying to apply the concept. The historical entries treat the term as self-explanatory to a readership trained in Roman law, which modern researchers should not assume.
Burrill's entry is notable primarily for its inclusion of the adjacent term Exquaestor in the same passage, suggesting the entry was drawn from a dense lexicon without editorial separation — a reminder to verify that dictionary pagination in older sources does not blend distinct terms.
Jurisdictional Note
Expromittere has no direct application in common law jurisdictions. It remains relevant to research in Louisiana, Quebec, South Africa, Scotland, and other mixed or civilian jurisdictions where Roman law concepts of novation and delegation survived reception. Civilian codes in these systems preserve the functional equivalent even when the Latin term itself is not used.