Flat, alluvial, often marshy land beside a river, estuary, or the sea. Now chiefly: specifically partially reclaimed land bordering the Solway Firth.
"the Merse": the fertile lowland region of south-eastern Scotland between the Lammermuir Hills and the Tweed (formerly in the county of Berwickshire; since 1975 in the Berwickshire district of Borders region).
Middle English. Variant of marsh; some forms show regular loss of -k in final -sk.