preimmune

The story

1. Proto-Indo-European*per-
2. Proto-Indo-European*preh₂-
3. Proto-Indo-European*préh₂i?
4. Proto-Italic*prai
5. Proto-Italic*prai-
6. Latinprae-
7. Middle Englishpre-
8. Englishpre-
9. Proto-Indo-European*mey-
10. Proto-Indo-European*moy-nós
11. Proto-Italic*moinos
12. Latinmūnus
13. Proto-Indo-European*né
14. Proto-Indo-European*n̥-
15. Proto-Italic*n̥-
16. Latinin-
17. Latinin- -is
18. Latinimmūnis
19. Middle Frenchimmun
20. Englishimmune
21. English todaypreimmune

From pre- + immune.