Where does “immortal” come from?

Immortal derives from Latin immortalis, composed of the negating prefix in- and mortalis, ultimately tracing to Proto-Indo-European ne meaning "not."

immortal (English): Not susceptible to death; living forever; never...

Definitions

Step	Language	Word	Meaning
1	Latin	immortālis	immortal, undying, not subject to death
2	Latin	im-	Alternative form of in-; form of "in"; in
3	Latin	īn-	un-, non-, not
4	Latin	în	in, at, on, upon, from (space)
5	Latin	en	lookǃ beholdǃ; reallyǃ?; come onǃ
6	Proto-Italic	en	in
7	Proto-Indo-European	h₁én	in