Where does “faïθ” come from?

faïθ (Proto-Brythonic) comes from Latin factum, from Proto-Indo-European dʰeh₁- — to do, put, place.

Ancestry of “faïθ”, step by step

Step	Language	Word	Meaning
1	Latin	factum	fact, deed, act, doing; achievement; accusative...
2	Proto-Indo-European	dʰeh₁-	to do, put, place

faïθ — Etymology, Origin & Word History | Etymology Explorer