Where does “Ш” come from?

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

Ancestry of “Ш”, step by step

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

Ш — Etymology, Origin & Word History | Etymology Explorer