Definition
To usurp or exercise power without lawful authority; to encroach upon the prerogatives of the Crown. The term appears primarily in medieval and early English law, where it described the act of arrogating royal power to oneself — behaving as though one possessed sovereign authority that belonged exclusively to the king. The concept carried severe legal consequences: conduct amounting to acoroach could be treated not merely as a civil wrong or ordinary crime, but as treason.
Common Confusion
ACOROACH and ENCROACH share a common root and overlapping meaning in the sense of unlawful intrusion, but they are not interchangeable in historical legal usage. Encroach describes the gradual, unauthorized extension of one's rights or property into another's domain — a concept still active in modern property law. Acoroach is narrower and more archaic: it specifically connotes the seizure or imitation of royal or sovereign power, with a treasonous dimension that ordinary encroachment does not carry. Researchers encountering either term in early English sources should not assume equivalence.
Why It Matters in Research
Acoroach is an obsolete term with no living presence in modern law. Its research value is entirely historical and confined to a narrow band of medieval and early modern English legal materials. Researchers working in the history of treason law — particularly the development of constructive treason under and following the Treason Act 1351 — will encounter the concept in that context. The term illustrates how broadly treason was defined in early English law: conduct that fell short of direct rebellion against the king could still be prosecuted as treason if it involved the assumption of royal authority.
The example preserved in Black's is instructive: a knight who forcibly detained a subject until payment was extracted was held to have committed treason on the theory of accroachment. This is not extortion in the modern sense — the treasonous element was the implicit claim to exercise coercive sovereign power, not merely the taking of money. Researchers tracing the history of extortion, duress, or ransom in English law should be aware of this framing, as it will affect how source materials characterize conduct that modern law would analyze under entirely different categories.
Spelling is unstable across historical sources. Black's 2nd Edition uses "acoroach," but the same concept appears as "accroach" and "accroachment" in Blackstone and Hale. Searches in digitized primary sources should account for all variants. The related noun form, accroachment, is more commonly encountered than the verb form.
Historical Dictionary Support
Black's Law Dictionary (2nd Edition) defines acoroach as "to encroach; to exercise power without due authority" and specifically "to attempt to exercise royal power," citing Blackstone's Commentaries (4 Bl. Comm. 76) and Hale's Pleas of the Crown (1 Hale, P.C. 80). The entry is brief but precise in its treasonous dimension, grounding the term in the specific historical example of a knight's forcible detention for ransom.
Black's treatment reflects the consensus of the historical legal dictionary tradition on this point: the term belongs to the law of treason, not merely to property or administrative law. No significant divergence exists among sources because the term was already archaic by the time modern legal dictionaries were compiled; they preserve it as a historical record rather than as an operative definition. What the historical dictionaries do not address is the broader doctrinal context — namely, how accroachment fit within the structure of constructive treason and why it eventually fell out of distinct use as treason law was refined and codified.
Jurisdictional Note
Acoroach is a term of English common law origin with no recognized counterpart in American, Scottish, or other common law jurisdictions. It does not appear in any operative modern legal framework.