The name stems from residence beside a moss (or marsh). Black tells us that Gregory de Mos was tenant of the Earl of Douglas of Lourchurde in 1376, among other findings.
Presumably, people living in the Maxwell-dominated area of the Solway Moss used this as their surname. Also, the shorelines of that area’s Solway Firth and River Nith undoubtedly have marshy areas from which people might have taken their name.
