"Or if the partie soe offending as aforesaid shall refuse or bee unable to recompense the party soe wronged, or to satisfy such fyne or forfeiture, then such Offender shalbe severely punished by publick whipping and imprisonment during the pleasure of the Lord Proprietary, or his Lieutenant or cheife Governor of this Province for the tyme being without baile or maineprise..."
--Maryland Toleration Act, 1649, reprinted in Donald S. Lutz, ed. Colonial Origins of the American Constitution: A Documentary History (Indianapolis: Liberty Fund, 1998).
OED lists this as an alternate spelling (among many others) of mainprize: "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."