Where does “in fieri” come from?

in fieri (Latin) comes from Latin in, from Old Latin en, from Proto-Italic en, from Proto-Indo-European h₁én — in.

Ancestry of “in fieri”, step by step

Step	Language	Word	Meaning
1	Latin	in	in, at, on, upon, from; within, while in; into,...
2	Old Latin	en	in
3	Proto-Italic	en	in
4	Proto-Indo-European	h₁én	in

in fieri — Etymology, Origin & Word History | Etymology Explorer