Where does “san-” come from?

san- (Latin) comes from Proto-Indo-European h₁sh₂-én-, from Proto-Indo-European h₁ésh₂r̥ — blood.

Ancestry of “san-”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	h₁sh₂-én-	—
2	Proto-Indo-European	h₁ésh₂r̥	blood