Anglo-French 'plaintif,' the complaining one — the party who brings a grievance to court, etymologically someone arriving with a lament.
A person who brings a case against another in a court of law; the party who initiates a civil lawsuit.
From Anglo-French 'plaintif' (complaining, aggrieved), used as a noun from the adjective meaning 'one who complains,' from Old French 'plaindre' (to lament, to complain), from Latin 'plangere' (to beat the breast in grief, to lament loudly), from PIE *plāk- (to strike, to beat). The plaintiff is literally the complaining party — the one who comes to court with a grievance, lamenting the wrong done to them. The same root produces 'plaint' (a complaint), 'complaint,' 'plaintive' (sorrowful, lamenting), and through Latin 'plangere,' the English 'plangent' (loud and mournful). Key
The plaintiff and the word 'plaintive' (sorrowfully mournful) are the same word. Medieval courts received the plaintiff as someone who came lamenting — literally beating their breast in grief over a wrong suffered. The sorrowful quality of the adjective 'plaintive' is not a metaphor but the original, physical meaning of the root: the act of striking oneself in mourning.