Wetwang was the name in Westron for the pathless fen beneath the Emyn Muil that the Elves called Nindalf. They lay to the south of Emyn Muil and east of the Great River Anduin, fed by the great inland delta of the Entwash. The Dead Marshes lie further east and may be an extension of Wetwang.
The archaic meaning of wang means 'field, flat area'.
Wetwang is a real-life place-name in Yorkshire.