Definition
FOS is an archaic abbreviation appearing in medieval Latin and Law French legal mnemonics, most notably in the couplet used to identify the courts competent to hear assizes of nuisance. In that context, FOS stands for FOSSE — the ditch, trench, or watercourse — as one of the recognized categories of physical encroachment or interference that could ground a nuisance action cognizable in a particular forum.
The full mnemonic couplet, as preserved in Burrill, runs:
"stag. sepe. que vi. diversi cursus aquarum, / Poscunt assisam; mercatum, feria, bancum."
Written out, the abbreviated terms identify specific types of nuisance — including stagnant water (stag.), hedge (sepe.), watercourses (diversi cursus aquarum), market (mercatum), fair (feria), and bank or bench (bancum) — each of which invoked a particular assize proceeding. FOS/FOSSE was embedded in this mnemonic tradition as shorthand for ditch-related nuisances: wrongful ditching, obstruction of drainage, or interference with established water flow across neighboring land.
The term does not carry independent doctrinal weight as a standalone legal concept. Its significance is procedural and jurisdictional: the type of nuisance determined which court held cognizance of the assize, and the mnemonic compressed that routing rule into a memorizable form for practitioners trained in oral and manuscript legal culture.
Common Language
Modern common usage (Wiktionary): Plural of FO (informal abbreviation, various contexts including "Forward Observer" in military usage or a given name).
Historical common usage: No entry in Webster's 1913 for FOS as a standalone word.
The gap here is total. Modern usage of FOS as an abbreviation bears no relation to its function in medieval legal sources, where it is a compressed rendering of FOSSE within a Latin jurisdictional mnemonic. A researcher encountering FOS in an early manuscript or printed law dictionary should not import any modern abbreviated meaning.
Common Confusion
FOS and FOSSE are easily conflated with each other and with the Roman and English legal concept of the FOSSA — the ditch as a boundary marker or element of fortification, carrying its own property and military law implications. The mnemonic context clarifies that FOS in this setting refers specifically to the watercourse or drainage ditch as a nuisance object, not to boundary demarcation or fortification. Researchers should also take care not to read FOS as FO'S (a possessive) or as a mistranscription of FON, FOSS, or similar abbreviations appearing in parallel manuscript traditions.
Why It Matters in Research
FOS will not appear as a headword in modern legal databases. Researchers will encounter it only in:
1. Early printed law dictionaries (Cowell, Blount, Burrill) that preserved or glossed the medieval mnemonic tradition.
2. Manuscript or early printed treatises on the assize of nuisance, where the couplet appears as a practitioner's memory aid.
3. Secondary literature on medieval English procedure, particularly treatments of the assize system before the forms of action were abolished.
The primary trap is misreading the abbreviation. Because the mnemonic compresses multiple Latin and Law French terms into single syllables or abbreviated fragments, a researcher who does not recognize the couplet's structure may misconstrue the surrounding text entirely. Burrill's entry itself is incomplete as printed — the entry breaks off mid-sentence — which means the full gloss must be reconstructed from Cowell or from the underlying legal sources the couplet references.
The jurisdictional intelligence embedded in the mnemonic is also genuinely useful for research into disseisin by nuisance: the type of physical interference (ditch, hedge, watercourse, market, fair, banking structure) mapped onto specific court competence, and understanding that mapping is necessary to follow medieval pleading records accurately.
Historical Dictionary Support
Burrill's Law Dictionary (the sole source dictionary here) treats FOS as an abbreviation within the mnemonic couplet rather than as an independent entry, and explicitly refers the reader to FOSSE for the substantive definition. Burrill traces the couplet to Cowell and to the Laws of William the Conqueror (L. Fr. Gul. Conq. 1. 30), signaling its deep antiquity. The Burrill entry is unfortunately truncated, cutting off before the full gloss is rendered.
Cowell's Interpreter, which Burrill cites, provides a fuller treatment of FOSSE and the assize framework. Blount's Nomo-Lexicon similarly preserves the mnemonic tradition. None of the standard modern legal dictionaries (Black's, Bouvier's later editions) carry this entry in any useful form, reflecting the complete obsolescence of the forms of action and the assize system in practice.
What the historical dictionaries collectively miss: they do not adequately explain why the mnemonic survived in printed dictionaries long after the assizes it described had ceased to function. For researchers in legal history, the mnemonic's persistence is itself evidence of the conservative transmission habits of the English legal dictionary tradition.
Jurisdictional Note
FOS and the mnemonic in which it appears are exclusively English in origin and application, reflecting the medieval English assize system. The concept has no counterpart in Scottish, Irish, or civilian legal traditions, and does not appear in American legal sources except as a historical curiosity reproduced from English authorities.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia: Nuisance — for the substantive doctrine governing watercourse and ditch interference.
See Law Mind Encyclopedia: Assize — for the procedural framework the mnemonic encodes.
See Law Mind Encyclopedia: Disseisin — for the broader dispossession context in which nuisance assizes arose.
See Law Mind Encyclopedia: Forms of Action — for the abolition of the system that rendered the mnemonic obsolete.