Where does “sost” come from?

sost (Old French) comes from Latin subtus, from Latin sub, from Proto-Italic supo, from Proto-Indo-European upó, from Proto-Indo-European h₃ewp-.

Ancestry of “sost”, step by step

Step	Language	Word	Meaning
1	Latin	subtus	below, underneath, beneath
2	Latin	sub	under, beneath; behind; at the feet of
3	Proto-Italic	supo	under
4	Proto-Indo-European	upó	under, below
5	Proto-Indo-European	h₃ewp-	—