There is actually a real place named Wetwang in Yorkshire, England, though that name has a slightly different derivation. Where the wet in Tolkien's Wetwang simply carries its natural meaning of 'wet' or 'watery' (as shown by its Elvish equivalent nin) the real place-name actually has nothing to do with water. Instead it derives from Old Norse vaétt, 'legal action', so the full name refers to a field where trials would take place.