Law French (Middle English: Lawe Frensch) is an archaic language originally based on Anglo-Norman, but increasingly influenced by Parisian French and, later, English. It was used in the law courts of England, since the 13th century.^[2] Its use continued for several centuries in the courts of England and Wales and Ireland. Although Law French as a narrative legal language is obsolete, many individual Law French terms continue to be used by lawyers and judges in common law jurisdictions.

The earliest known documents in which 'French' (i.e. Anglo-Norman) is used for discourse on English law date from the third quarter of the thirteenth century and include two particular documents. The first is The Provisions of Oxford^[3] (1258), consisting of the terms of oaths sworn by the 24 magnates appointed to rectify abuses in the administration of King Henry III, together with summaries of their rulings. The second is The Casus Placitorum^[4] (c. 1250 – c. 1270), a collection of legal maxims, rules and brief narratives of cases.

In these works the language is already sophisticated and technical, well equipped with its own legal terminology. This includes many words which are of Latin origin but whose forms have been shortened or distorted in a way which suggests that they already possessed a long history of French usage. Some examples include advowson from the Latin advocationem, meaning the legal right to nominate a parish priest; neif[e], from the Latin nātīvā, meaning a female serf; and essoyne or essone from the Latin sunnis, meaning a circumstance that provides exemption from a royal summons (later essonia replaced sunnis in Latin, thus replacing into Latin from the French form).

Until the early fourteenth century, Law French largely coincided with the French used as an everyday language by the upper classes. As such, it reflected some of the changes undergone by the northern dialects of mainland French during the period. Thus, in the documents mentioned above, 'of the king' is rendered as del rey, or del roy, whereas by about 1330 it had become du roi (as in modern French) or du roy.^[5][6]

During the 14th century vernacular French suffered a rapid decline. The use of Law French was criticized by those who argued that lawyers sought to restrict entry into the legal profession. The Pleading in English Act 1362 ("Statute of Pleading") acknowledged this change by ordaining that thenceforward all court pleading must be in English so "every Man ... may the better govern himself without offending of the Law".^[7] From that time, Law French lost most of its status as a spoken language. It remained in use for the 'readings' (lectures) and 'moots' (academic debates), held in the Inns of Court as part of the education of young lawyers, but essentially it quickly became a written language alone; it ceased to acquire new words, its grammar degenerated (by about 1500 gender was often neglected, giving rise to such absurdities as une home ('a (feminine) man') or un feme ('a (masculine) woman'), and its vocabulary became increasingly English, as it was used solely by English, Welsh and Irish lawyers and judges who often spoke no real French.

In the seventeenth century, the moots and readings fell into neglect, and the rule of Oliver Cromwell, with its emphasis on removing the relics of archaic ritual from legal and governmental processes, struck a further blow at the language. Even before then, in 1628, Sir Edward Coke acknowledged in his preface to the First Part of the Institutes of the Law of England that Law French had almost ceased to be a spoken tongue. It was still used for case reports and legal textbooks until almost the end of the century, but only in an anglicized form. A frequently quoted example of this change comes from one of Chief Justice Sir George Treby's marginal notes in an annotated edition of Dyer's Reports, published 1688:

Richardson Chief Justice de Common Banc al assises de Salisbury in Summer 1631 fuit assault per prisoner la condemne pur felony, que puis son condemnation ject un brickbat a le dit justice, que narrowly mist, et pur ceo immediately fuit indictment drawn per Noy envers le prisoner et son dexter manus ampute et fix al gibbet, sur que luy mesme immediatement hange in presence de Court.

("Richardson, Chief Justice of the Common Bench at the Assizes at Salisbury in Summer 1631 was assaulted by a prisoner there condemned for felony, who, following his condemnation, threw a brickbat at the said justice that narrowly missed, and for this, an indictment was immediately drawn by Noy against the prisoner and his right hand was cut off and fastened to the gibbet, on which he himself was immediately hanged in the presence of the Court.")^{[8][note 1]}

Only the Proceedings in Courts of Justice Act 1730 made English (instead of Law French and Latin) the obligatory language for use in the courts of England and in the court of exchequer in Scotland. It was later extended to Wales, and seven years later a similar act was passed in Ireland, the Administration of Justice (Language) Act (Ireland) 1737.^[9]

The postpositive adjectives in many legal noun phrases in English—attorney general, fee simple—are a heritage from Law French. Native speakers of French may not understand certain Law French terms not used in modern French or replaced by other terms. For example, the current French word for "mortgage" is hypothèque. Many of the terms of Law French were converted into modern English in the 20th century to make the law more understandable in common-law jurisdictions. However, some key Law French terms remain, including the following:

• French language
• Norman language
• French phrases used by English speakers
• English words of French origin
• Jersey Legal French
• Franglais
• List of legal Latin terms
• Legal English