Plaintiff
late Middle English: from Old French plaintif ‘plaintive’ (used as a noun). The -f ending has come down through Law French; the word was originally the same as plaintive .
wiktionary
From Middle English plaintif, from Anglo-Norman, from Old French plaintif(“complaining; as a noun, one who complains, a plaintiff”) from the verb plaindre. Doublet of plaintive.
etymonline
plaintiff (n.)
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.
