Middle English. From Anglo-Norman mareis, marreis, marrais, mereis, mareschei and Middle French mares, marez, marais, marest marsh from post-classical Latin mariscus, marisca, marescus, ultimately from the Germanic base of marsh. Compare post-classical Latin maresium, maresia (frequently from 1130 in British sources). Compare Anglo-Norman maresche marshy.

Late 17th century; earliest use found in The London Gazette. From mare + -ish.