Where does “in-” come from?

in- (Spanish) comes from Latin īn-, from Latin în, from Latin en, from Proto-Italic en, from Proto-Indo-European h₁én — in.

in- (Spanish): not

Definitions

Step	Language	Word	Meaning
1	Latin	īn-	un-, non-, not
2	Latin	în	in, at, on, upon, from (space)
3	Latin	en	lookǃ beholdǃ; reallyǃ?; come onǃ
4	Proto-Italic	en	in
5	Proto-Indo-European	h₁én	in