Where does “sí” come from?

sí (Irish) comes from Old Irish sí, from Proto-Celtic sī, from Proto-Indo-European sih₂, from Proto-Indo-European -ih₂, from Proto-Indo-European -h₂ — Creates collective nouns, which refer to groups...

sí (Irish): fairy mound, tumulus; she; it

Definitions

fairy mound, tumulus; she; it

Ancestry of “sí”, step by step

sí traces back through two separate lines of ancestry.

via Old Irish sí

Step	Language	Word	Meaning
1	Old Irish	sí	she; it; Alternative spelling of síi
2	Proto-Celtic	sī	she
3	Proto-Indo-European	sih₂	she
4	Proto-Indo-European	-ih₂	Feminizes athematic nominal stems
5	Proto-Indo-European	-h₂	Creates collective nouns, which refer to groups...

via Middle Irish side

Step	Language	Word	Meaning
1	Middle Irish	side	fairy hill or mound
2	Old Irish	síd	fairy mound; fairy people, fairies; peace,...
3	Proto-Celtic	sīdos	tumulus; peace
4	Proto-Indo-European	sēds	—

Words derived from “sí”

Every word from Proto-Indo-European -h₂ →Every word from Proto-Indo-European -ih₂ →Every word from Proto-Indo-European sih₂ →