The action of procuring the release of a prisoner on someone's undertaking to stand surety (‘mainpernor’) for his or her appearance in court at a specified time. Chiefly in phrases "to let (also receive) to (also in) mainprize", "to deliver upon (also by) mainprize", "to nim under mainprize". "without bail or mainprize": with no permission to obtain release by finding sureties.
Middle English; earliest use found in Statutes of the Realm. From Anglo-Norman mainprise, meinprise from mainprendre.