Where does “cúis” come from?

cúis (Middle Irish) comes from Old Irish caus, from Latin causa, from Old Latin caussa, from Proto-Indo-European kawd-teh₂.

Ancestry of “cúis”, step by step

Step	Language	Word	Meaning
1	Old Irish	caus	—
2	Latin	causa	cause, reason; case, claim, contention; motive,...
3	Old Latin	caussa	cause, reason, purpose; case, claim; Archaic form...
4	Proto-Indo-European	kawd-teh₂	—

cúis — Etymology, Origin & Word History | Etymology Explorer