early 13c., ple, "lawsuit, legal conflict," also "strife, contention, complaint," from Anglo-French plai (late 12c.), Old French plait "lawsuit, decision, decree" (9c.), from Medieval Latin placitum, plactum "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).
The sense development seems to have been from "something pleasant," to "something that pleases both sides," to "something that has been decided." Meaning "an entreaty, a pleading, an argument in a suit" is attested from late 14c. Plea-bargaining is attested by 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 "public prosecution in criminal cases"); later "actions brought by one subject against another."