Where does “úachtar” come from?

úachtar (Old Irish) comes from Proto-Celtic *ouxteros, from Proto-Indo-European h₃ewp-s- — high.

úachtar (Old Irish): surface, top; cream

Definitions

Step	Language	Word	Meaning
1	Proto-Celtic	*ouxteros	—
2	Proto-Indo-European	h₃ewp-s-	high

úachtar — Etymology, Origin & Word History | Etymology Explorer