Origins
The plaintiff is, etymologically, a person in mourning β or at least a person loudly voicing a grievance.βββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ The word comes from Anglo-French 'plaintif,' an adjective meaning 'complaining' or 'aggrieved' that was pressed into use as a noun to describe the party who initiated legal proceedings. Behind it lies Old French 'plaindre' (to lament, to complain), which descends from Latin 'plangere,' a verb denoting the physical act of beating the breast in grief β the loud, embodied mourning of antiquity, in which sorrow was expressed through striking one's own chest or cheeks.
Latin 'plangere' traces to the Proto-Indo-European root *plΔk- (to strike, to beat flat), a root also found in Latin 'plaga' (a blow, a stroke, a wound), from which English borrowed 'plague' β originally a devastating blow or stroke inflicted on a people, not specifically a disease. The same PIE root produced, through the idea of striking flat, the Old French 'flaon' (a flat cake, beaten or pressed into shape), which entered English as 'flan.'
The related words in English from the same Latin 'plangere' cluster around lamentation and mourning. 'Plaint' is a formal complaint or lamentation, used in legal writing ('a bill of complaint,' originally 'plaint'). 'Plaintiff' is the noun formed from the adjective 'plaintif.' 'Plaintive' retains the original emotional content most clearly β a plaintive sound is sorrowful, lamenting, mournful. 'Plangent' (used of sounds that are loud, resonant, and sorrowful) comes directly from the Latin present participle 'plangens.' 'Complaint' and 'complain' come from the same Old French source, with the prefix 'com-' adding a sense of intensity.
Later History
In English common law, the plaintiff was the party who brought a civil action β as distinct from the Crown, which prosecuted criminal cases, and as distinct from the defendant, who answered the claim. The formal documents of civil pleading began with the 'declaration' or, in earlier practice, the 'plaint,' setting out the plaintiff's grievance. The word encoded the assumption that the initiating party was genuinely aggrieved, lamenting a real wrong β though of course the truth of the grievance was exactly what the court had to determine.
In modern American practice, 'plaintiff' is the standard term in civil litigation. English law increasingly uses 'claimant' (introduced by the Civil Procedure Rules of 1999 to replace 'plaintiff' in most civil contexts), on the grounds that the word is more descriptively accurate and less archaic. The change is telling: 'claimant' is neutral, bureaucratic, precise; 'plaintiff' carries centuries of lamentation and grief inside it. The etymological mourner at the breast has been replaced by a bureaucratic claimant, but the older word persists in American courts, in legal history, and wherever the Anglo-French legal tradition continues to make itself heard.