Now, we might just be simple, down-to-earth dictionary folk from Massachusetts, but we like to think we know a thing or two about words and how they work. And we couldn't help but notice that lawsuit and complaint are sometimes misused in the media and journalism.
By definition, lawsuit refers to the legal process (that is, the court case) by which a court of law makes a decision on an alleged wrong (as exhibited in the statement "a complex lawsuit that may take years to resolve"), whereas complaint refers to the initial document, or pleading, submitted by a plaintiff against a defendant that details how the plaintiff's legal rights were violated in some way and that sets forth the plaintiff's demand for relief, monetary or otherwise. In other words, complaint refers to the set of papers that outlines the following lawsuit. It can vary in length, and it sets the stage for the courtroom drama that is about to follow. (People of a certain age might recall a Judge Wapner's greeting to the litigants on "The People's Court" in which he says, "I know you've been sworn. I've read your complaint."—note that he did not say "I've read your lawsuit.")
Lawsuit is etymologically sewn to suit, and suit itself has a suite of diverse meanings in law, fashion, romance, and card playing. The word ultimately derives, via Anglo-French suite, from Vulgar Latin sequitus, the past participle of sequere, meaning "to follow," which is also a descendant of the verb sue. Knowing this etymology might help you (if you are guilty of misuse) differentiate between lawsuit and complaint since a lawsuit follows a complaint.
Before suit came to refer to a legal action, it had various senses in Middle English referring to acts of following or pursuing, literally and figuratively. Early uses of suit refer to the required attendance by a tenant at his lord's court as well as to a company of followers in general.
... had there not come in Tydeus and Telenor, with forty or fifty in their suit, to the defense of Plexirtus.
— Philip Sidney, The Countess of Pembroke's Aracadia, 1590
Suit was also used in Middle English to refer to the pursuit of game—later, to the pursuit of a criminal on the run.
Freshsuit, is when a man is robbed, and the partye so robbed, followeth the felon immediatlye.
— John and William Rastell, An exposition of certaine difficult and obscure words and termes of the lawes of this realme, 1579
In the 16th century, the word came to designate a lover's persistent effort to win one's heart's desire as a suitor and, understandably, being a suitor involved following.
Reveale any more his sute he durst not, because when he began to chat of love she shakt him off.
— Robert Greene, Greenes never too late, 1590
Senses of the word referring to the pursuit of justice were developed in the 14th century, when a tenant had to appeal to a superior for justice. The exact evolution of the legal sense is unclear; however, it may have been influenced by the obligation of a tenant to be "in suit" ("in attendance") at the court of a lord. The "clothing" sense of suit is also connected to the feudal court. Those "in suit" at the court followed a certain dress code. That sense evolved to refer to articles of matching clothing (like a bathing suit) as well as to sets of other matching things (like the suits in a deck of cards). As you can see, suit has a history of "following"—and in the halls of justice, this is also the case.
Evidence for complaint attests that it derives, via Anglo-French, from the Latin verb plangere, meaning "to lament." It can refer to expressions of grief and protest in addition to legal allegations—so we have "stomach complaints," "customer complaints," and "legal complaints." Plaintiff is distantly related to the Latin verb, and, certainly, a plaintiff can be said to be complaining to the court that their legal rights have been violated in some way. We can't say our legal rights have been violated, but if it pleases the court we would like to submit as evidence the following misuse of lawsuit.
The nearly 100-page lawsuit claims the pharmaceutical giant, which has earned $35 billion in opioid manufacturing and sales, took on a marketing campaign aimed at creating doubt that opioids should be used sparingly.
— USA Today, 11 Sept. 2018The 92-page lawsuit, filed late Friday, alleges that Mitsubishi Aircraft and its Seattle contractor, AeroTEC, have hired about 92 former Bombardier personnel, some after holding job fairs near its Quebec headquarters and its U.S. flight test center in Wichita, Kansas.
— The Seattle Times, 19 Oct. 2018
Consider this our complaint. But not our last hackneyed movie courtroom reference. That one's coming now: we rest our case.