Guith, pronounced "gwith" is an old Brythonic word meaning separated, where Brythonic is the language spoken in Britain before the coming of the English. This is interesting as it helps ... explain two particular place names in England. The first of these is Ynis Gwith, the "separated island", which is the Isle of Wight which lies off the south coast of England; the second is Pen Gwith or Penwith, the "separated headland", which is the name given to the most westerly district of Cornwall, the very tip of the Cornish peninsula.

In both cases they were "separated" because people once thought they had been joined to something else.
In the case of the Isle of Wight this meant that people believed that it had once been joined to the mainland. As far as Penwith was concerned it was believed that it had once been joined to a larger land mass to the west, which is to say Lyonesse, the country beyond Land's End.