plaintive (adj.)

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 with plaintive at first; the form that receded into legal usage retained the older -iff spelling.