early 13c., "lawsuit," from Anglo-French plai (late 12c.), Old French plait "lawsuit, decision, decree" (9c.), from Medieval Latin placitum "lawsuit," in classical Latin, "opinion, decree," literally "that which pleases, thing which is agreed upon," properly neuter past participle of placere "to please, give pleasure, be approved" (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 late 14c. Plea-bargaining is first attested 1963. Common pleas (early 13c.) 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."