Where does “упо̀слити” come from?

упо̀слити (Serbo-Croatian) comes from Serbo-Croatian у, from Proto-Slavic vъ(n), from Proto-Indo-European h₁n̥, from Proto-Indo-European h₁én — in.

Ancestry of “упо̀слити”, step by step

Step	Language	Word	Meaning
1	Serbo-Croatian	у	in, at; to, into; on, in, at, during
2	Proto-Slavic	vъ(n)	in, inside, within; in, into, inside; at
3	Proto-Indo-European	h₁n̥	—
4	Proto-Indo-European	h₁én	in

упо̀слити — Etymology, Origin & Word History | Etymology Explorer