plaintiff (n.)

c. 1200, pleinte, "lamentation, mourning, audible expression of sorrow," from Old French plainte "lament, lamentation" (12c.), from Latin planctus "lamentation, wailing, beating of the breast," from past-participle stem of plangere "to lament; to strike (the breast, in grief or mourning)," from PIE root *plak- (2) "to strike." The connecting notion in Latin probably is beating one's breast in grief. Meaning "complaint, murmuring, grumbling" is from late 14c. Sense of "lawsuit, legal complaint, statement of grievances made to a court for the purpose of asking redress" is from late 14c.

late 14c., "lamenting, complaining, giving utterance to sorrow or grief," from Old French plaintif "complaining; wretched, miserable," from plainte (see plaint). Sense of "expressive of sorrow or melancholy, mournful, sad" is recorded from 1570s. Earlier was pleintful "grievous, lamentable" (early 14c.). Related: Plaintively; plaintiveness.