All Those Painted Men of The North

You wouldn't know by looking, but the words Cruithne and Prydain are, in fact, cognates.

It seems that humans sometimes have a tendency to migrate the leading labio-velar consonants of their languages into unvoiced bilabial consonants — in other words, the K^w sound at the beginning of quark becomes a P sound, as in park. At some point prior to the medieval era, this happened in the insular Celtic languages. The Celts of what is now Ireland retained the leading K^w sound in their words, while the Celts of what is now Wales transformed it into a P sound; linguists call these two branches of the same root language Q Celtic and P Celtic, respectively.

The modern word Britain comes from a P Celtic antecedent of the modern Welsh word, Prydain (pronounced PRIH-dine). Cruithne (pronounced somewhat like CROON-ya) is a modern Irish word derived from Prydain's Q Celtic equivalent.

Both of these words, and their parent word, are understood to mean something akin to "people of forms" — the Gaelic root, cruth, and the Welsh root, pryd, coming from the reconstructed proto-Celtic k^writu-, "shape, appearance" — and this is understood to mean people who tattooed or war-painted or otherwise decorated their bodies.

—L.
CCR