Where does “improprius” come from?

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

improprius (Latin): not proper; improper

Definitions

Step	Language	Word	Meaning
1	Latin	in-	un-, non-, not; in, within, inside
2	Latin	in	in, at, on, upon, from; within, while in; into,...
3	Old Latin	en	in
4	Proto-Italic	en	in
5	Proto-Indo-European	h₁én	in

improprius — Etymology, Origin & Word History | Etymology Explorer