The Latin “prosequi” meant “to follow after, accompany, chase, pursue, attack, assail, or abuse.” It is composed of the prefix “pro” meaning “forward” and “sequi,” meaning “follow,” in the same sense as “sequel.” In its most literal sense, prosequi means, “follow forward.” In Latin, prosequi was usually used in two ways: literally to mean “follow a path” or figuratively to mean “follow a course of action.” The past participle of prosequi is “prosecutus.” Prosequi became a Latin agent noun “prosecutor” in medieval times. Prosecutus became “prosecute” in the early 15th century, usually meaning “to go into detail.” The first recorded use of the word to mean “bring to a court of law” is in the 1570s. At this time, the person who brought a case in a court of law was a “promoter.” Prosecutor, in turn, acquired its modern meaning in the 1620s.
“Plaintiff” by contrast, arose in the 14th century from the Anglo-French “pleintif,” which in turn was a noun made from the old French adjective “plaintif” meaning “complaining; wretched, or miserable.” At first, Plaintiff was used interchangeably with “plaintive,” but eventually the modern form prevailed.
Prosecutors are often called a “District Attorney” or “DA,” as I do on this blog. This title originated with the federal government. The United State Judiciary Act of 1789, Section 35, provided for the appointment of a person in each of the 13 existing judicial districts to represent the United States in civil suits. These judicial districts were not coextensive with counties or states, but rather their own geographical entities. The Act did not confer a title on these people, but court decisions and subsequent laws referred to them most frequently as “district attorneys.” States followed the federal government and appointed prosecutors to multi-county judicial districts. For example, New York kept up this practice until 1813. Even after those states broke up such districts and started appointing or electing prosecutors for individual counties, they continued to use the title “district attorney” for the most senior prosecutor in a county rather than switch to “county attorney.” Ironically, the federal government, which started the use of the title “district attorney” abandoned it and adopted the term “United States Attorneys” in 1948.