Where does “infallible” come from?

Infallible comes from Latin infallibilis, formed from the prefix in- and fallibilis, ultimately deriving from Proto-Indo-European ne meaning not.

infallible (English): Without fault or weakness; incapable of error or...

Definitions

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