The word "Folde" is the genitive singular of the Old English word "Foldan", which in turn is a feminine noun. It meaning can be translated to "of the earth", or simply "earth" or "land".
http://www.utexas.edu/cola/centers/lrc/eieol/engol-8-X.html
http://www2.thphy.uni-duesseldorf.de/~ansgar/languages.shtml
In that regards, it should perhaps be made clear as to why the word "Folde" was used by Tolkien, rather than leaving the page to mislead viewers that the "Folde" of Rohan (namely the Westfold and Eastfold as a collective) was invented by Tolkien and originally derived from Middle-Earth.