Where does “-ㅅ-” come from?

-ㅅ- (Korean) comes from Old Korean 叱 — Genitive case marker, chiefly for inanimate nouns.

Ancestry of “-ㅅ-”, step by step

Step	Language	Word	Meaning
1	Old Korean	叱	Genitive case marker, chiefly for inanimate nouns

-ㅅ- — Etymology, Origin & Word History | Etymology Explorer