The dialect of the Forest is definitely derived from the plain English of Chaucer, with no frills of Norman French.
Exceptions to this generalisation are found in a few place-names and in connection with the old handicrafts,
indicating that certain places and trades were developed under Celtic influence.
As an example we may take May Hill.
This is often called Yarleton, and the western slope is liberally sprinkled with elaborations of the Yarleton theme.
Efforts to explain this name have even gone to the length of deriving it from yaffle, or green woodpecker,
but they have shied at siting (citing? - EH) the Saxon ton to which this loud-voiced creature is alleged
to be linked.
But if we listen to the old folks we shall hear the name is Yarledon, or Iarkledun, which has a most convincing
resemblance to the Celtic iarkledune, meaning round-topped hill. Could any name be more likely from people with
a natural aptitude for descriptive names?
The above text is from The Forest of Dean by F. W. Baty
(publisher: Robert Hale 1952)