Origin and history of plaintive

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.

in law, "the person who begins a suit before a tribunal for the recovery of a claim" (opposed to defendant), c. 1400, pleintif, from Anglo-French pleintif (late 13c.), from noun use of Old French plaintif "complaining; wretched, miserable," in law, "aggrieved" (as in partie plaintif "the party bringing a suit at law"), from plainte (see plaint). Identical to plaintive at first; the form that receded into legal usage retained the older -iff spelling.