Where does “su” come from?

su (Old Irish) comes from Proto-Celtic su-, from Proto-Indo-European h₁su-, from Proto-Indo-European h₁wes- — good; to be cheerful, happy; to eat, to feast.

Ancestry of “su”, step by step

StepLanguageWordMeaning
1Proto-Celticsu-good, well
2Proto-Indo-Europeanh₁su-good
3Proto-Indo-Europeanh₁wes-good; to be cheerful, happy; to eat, to feast
Every word from Proto-Indo-European h₁wes- →Every word from Proto-Indo-European h₁su- →Every word from Proto-Celtic su- →
su — Etymology, Origin & Word History | Etymology Explorer