Where does “ש” come from?

ש (Hebrew) comes from Proto-Semitic šinn-, from Proto-Afroasiatic sin-.

Ancestry of “ש”, step by step

Step	Language	Word	Meaning
1	Proto-Semitic	šinn-	tooth
2	Proto-Afroasiatic	sin-	—

ש — Etymology, Origin & Word History | Etymology Explorer