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 us 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.

From the entry for;

Antivectarum promontorium
Land's End, Penwith, Cornwall

Förster cited Nennius on the Isle of Wight 'quam Britones insulam Gueid vel Guith, quod latine divorcium dici potest'. Latin divortium is 'a separation, a place where a road divides, a watershed'.
The answer to the meaning of wiht/with was right there all the time, the with or guith is the same as that of Nennius 'quam Britones insulam Gueid vel Guith'.'s%20End.htm

The Isle of Wight was known to the ancient Britons as Guith or Guiet, to the Romans as Vecta or Vectis, to the Saxons as Wiht, Whit, or Wight. The ancient British name signifies divided or separated, and is supposed to indicate that the island was dissevered from the mainland by the gradual formation of the Solent england/IsleofWight/gazetteer.html

the Romans turned this ... Isle Guith or Wight into Victesis or Vecta ... For Ninius hath taught us that guith in the British tongue betokeneth a separation

(That's enough sources. Ed)