Where does “mordeithiau” come from?

mordeithiau (Welsh) comes from Welsh mordaith, from Welsh môr, from Proto-Brythonic mọr, from Proto-Celtic mori, from Proto-Indo-European móri, from Proto-Indo-European mer- — to die; to disappear; sea, lake, wetland.

Ancestry of “mordeithiau”, step by step

mordeithiau traces back through two separate lines of ancestry.

via Welsh mordaith

Step	Language	Word	Meaning
1	Welsh	mordaith	sea voyage or cruise
2	Welsh	môr	sea, ocean, the deep; plenty, abundance,...
3	Proto-Brythonic	mọr	great, large
4	Proto-Celtic	mori	sea
5	Proto-Indo-European	móri	sea; standing water
6	Proto-Indo-European	mer-	to die; to disappear; sea, lake, wetland

via Welsh Iau

Step	Language	Word	Meaning
1	Welsh	Iau	Jupiter, Thursday
2	Middle Welsh	ieu	—
3	Brythonic	*Yow	—
4	Latin	Iovem	—
5	Proto-Indo-European	dyew-	to be bright; sky, heaven

Every word from Proto-Indo-European mer- →Every word from Proto-Indo-European móri →Every word from Proto-Celtic mori →