From Tolkien Gateway

Fenmarch was a region of eastern Rohan that stretched from the Firien Wood along the Mering Stream, and therefore marked part of Rohan's eastern border with Gondor.[1]


The fen element of the name means "marsh" and is a reference to the wetlands along the Mering Stream. March is an old word for "borderland" - a reference to the border with Anórien - and has the same derivation as Mark, a name for the whole country of Rohan.[2]

Note that Tolkien writes that on reflection he would have preferred the form "Fenmark", to the extent that he recommended this alternative version for translated versions of the book.[2]