Origin: 1175–1225; Middle English ple, earlier plaid < Old French < early Medieval Latin placitum law-court, suit, decision, decree, Latin: opinion (literally, that which is pleasing or agreeable), noun use of neuter of past participle of placēre to please
a. law something alleged or pleaded by or on behalf of a party to legal proceedings in support of his claim or defence
b. criminal law the answer made by an accused to the charge: a plea of guilty
c. (in Scotland and formerly in England) a suit or action at law
3.
an excuse, justification, or pretext: he gave the plea of a previous engagement
[C13: from Anglo-Norman plai, from Old French plaid lawsuit, from Medieval Latin placitum court order (literally: what is pleasing), from Latin placēre to please]
c.1215, "lawsuit," from Anglo-Fr. plai (c.1170), O.Fr. plait "lawsuit, decision, decree" (842), from M.L. placitum "lawsuit," in classical L., "opinion, decree," lit. "that which pleases, thing which is agreed upon," properly neut. pp. of placere (see please). Sense development
seems to be from "something pleasant," to "something that pleases both sides," to "something that has been decided." Meaning "a pleading, an agreement in a suit" is attested from c.1381. Plea bargaining is first attested 1963. Common pleas (c.1215) originally were legal proceedings over which the Crown did not claim exclusive jurisdiction (as distinct from pleas of the Crown); later "actions brought by one subject against another."