Definition
Latin: "accustomed to bite." A term of pleading used in common law actions for injury caused by a dangerous dog. When a plaintiff sought damages for harm done by a dog — whether to a person or another animal — the declaration was required to allege that the dog was *ad mordendum assuetus*, meaning that the owner knew, or was on notice, that the animal had a propensity to bite. The phrase thus carried the legal weight of the scienter allegation: without it, or its equivalent, the plaintiff's pleading was defective.
Common Confusion
Ad mordendum assuetus is sometimes loosely equated with the so-called "one bite rule," but the two are not identical. The Latin phrase is a pleading term — a formal averment required in the declaration — while the "one bite rule" is a shorthand for the underlying substantive doctrine that an owner is not liable for a first, unprovoked bite absent prior notice of the animal's dangerous tendency. The phrase establishes *how* that notice was pleaded, not the rule itself.
Why It Matters in Research
Researchers working in early American or English common law tort materials will encounter this phrase as a fixed formula in dog-bite declarations. Its significance is primarily procedural: courts treated the averment as material, meaning its omission could be fatal to a plaintiff's pleading. This makes the phrase a reliable diagnostic when scanning historical declarations or pleading books — its presence signals a scienter-based animal liability claim rather than a strict liability theory.
The phrase also marks a jurisdictional and temporal boundary. As American states moved through the nineteenth and twentieth centuries toward strict liability statutes for dog bites, the need for this specific averment diminished and eventually disappeared from practice. Researchers tracing the evolution from scienter-based to strict liability dog-bite law will find ad mordendum assuetus at the starting point of that transition. Corpus searches for the phrase will surface common law pleading materials and early case law, but will not capture the later statutory regime — a gap that matters when assessing the full arc of animal liability doctrine.
The phrase appears in pleading treatises rather than in substantive doctrinal works, which means researchers focused on the underlying law of animal liability may overlook it entirely if they work only through hornbooks and digests rather than practice manuals.
Historical Dictionary Support
Black's (1st and 2nd editions) and Burrill are in full agreement on substance, all three citing Croke's *Caroli* reports (Cro. Car. 254) and Chitty's *Pleading* (1 Chit. Pl. 388; 2 Chit. Pl. 597) as authority. The entries are essentially identical in content, differing only in minor typographical variation — Black's 2nd edition contains an apparent misprint ("1 Chit. Pl. 888" for 388). None of the historical dictionaries address the doctrinal context in depth or explain how the phrase fits within the broader structure of scienter liability, treating it purely as a pleading term. Researchers should not rely on these entries alone to understand the substantive law the averment was designed to support.
Jurisdictional Note
The phrase is rooted in English common law pleading and carried into early American practice. Its relevance is essentially historical; modern American dog-bite law in most states is governed by strict liability statutes that do not require a scienter averment, rendering the phrase obsolete in contemporary pleading. Researchers in jurisdictions that retain common law negligence theories for animal attacks may find the doctrine underlying the phrase still analytically relevant, even where the Latin formula itself is no longer used.