Where does “seiniau” come from?

seiniau (Welsh) comes from Welsh Iau, from Middle Welsh ieu, from Brythonic *Yow, from Latin Iovem, from Proto-Indo-European dyew- — to be bright; sky, heaven.

Ancestry of “seiniau”, step by step

seiniau traces back through two separate lines of ancestry.

via Welsh Iau

StepLanguageWordMeaning
1WelshIauJupiter, Thursday
2Middle Welshieu
3Brythonic*Yow
4LatinIovem
5Proto-Indo-Europeandyew-to be bright; sky, heaven

via Welsh sain

StepLanguageWordMeaning
1Welshsainsound
Every word from Proto-Indo-European dyew-Every word from Latin IovemEvery word from Brythonic *Yow