mid-13c., pleden, "make a plea in court," from Anglo-French pleder, Old French plaidier, "plead at court" (11c.), from Medieval Latin placitare, from Late 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).
From mid-14c. as "advance (something) as evidence, cite (something) in support of an action or in response to a complaint." Sense of "request, beg" is recorded from c. 1400. Related: Pleaded; pleading; pleadingly.