Celtic meaning of five. Supposedly four kingdoms surrounding a smaller central one (alternatively seen as being made up from a portion of each) where the overking resides.
Known are North Welsh ('The Men of the North' Yr Gogledd inc. Cumberland 'land of our brother Welsh' and Westmoreland), South Welsh (present Wales), West Welsh (Dumnonia, the West Country). East Welsh taken over very early on, all those Angles and such. Welsh heartland probably where Arthur came out of, so they made a thorough job of wiping out any traces there.