Uh oh... here comes the bad thread idea remorse...

Yes Stag_Beetle, I quite agree. There are many words from the ancient language of Brythonic ( which predates " Old English," it was the language of King Arthur) that are common to all three of the branches of Gaelic, ( Welsh, Irish and Scottish). Fay ( faegh) is but one.

Fay has the connotation of being connected with the gods, much as the Greek heroes were in their various mythological cycles. Therefore its ancient meaning is not at all consistent with the modern meaning of "doomed to die." Its deeper meaning has to do with living with the energy of the gods of that ancient world, and incorporating their energy into everyday life. Some individuals were talented at this and as a result were sometimes unpredictable in their conduct. This is the root meaning of "fey." Modern men and women are fascinated with the associated fearlessness of many of these ancient ones, who were often fearless in the face of death. They lived with each foot in different worlds, the human realm, susceptible to it's various impermanences, and the realm of the gods, which knows no passage of time or impermanence.

Fey is a very important word. It is a gateway to our past.

Fey would seem to be a native English word. See Etymology On Line's entry. If it is common to the British Isles' Celtic languages it is probably because they got it from English. To say that they Brythonic languages predate Old English does not draw a good picture. The Goidelic (Irish and Scots Gaelic and Manx) and Brythonic (Welsh and Cornish) languages are in a completely different branch of the Indo-European languages from the Germanic group, of which English is an example.

The Goidelic (Irish and Scots Gaelic and Manx) and Brythonic (Welsh and Cornish) languages

Let's not forget Breton, also of the Brythonic branch. There is another branch, the Continental, which includes Gaulish, and perhaps the other Celtic languages in Spain and Northern Italy / Southern Switzerland (e.g., Lepontic).

As for fey being Celtic. Do you have the root? Or any reflexes in individual Celtic languages? Fay in the sense of "fairy" is via Middle English from Old French. Fey, in the sense of "fated to die, doomed", is from Old English fǣge. "Fate, doom, destiny" in Welsh is tynged.

I didn't forget Breton. I was limiting myself to the languages of the British Isles.

I didn't forget Breton. I was limiting myself to the languages of the British Isles.

Fair enough. It was called Breton because the speakers were refugees from Britain.