Where does “infallibilis” come from?

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

Ancestry of “infallibilis”, step by step

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