Is it farther or further?
late 13c., "the carrying on of a suit at court," verbal noun from plead (v.). Meaning "supplication, intercession" is from early 15c.
mid-13c., "make a plea in court," from Anglo-French pleder, Old French plaidier, "plead at court" (11c.), from Medieval Latin placitare, from Late Latin placitum (see plea). Sense of "request, beg" first recorded late 14c. Related: Pleaded; pleading; pleadingly.
in law, written presentation by a litigant in a lawsuit setting forth the facts upon which he claims legal relief or challenges the claims of his opponent. A pleading includes claims and counterclaims but not the evidence by which the litigant intends to prove his case.