intercompartmental

The story

1. Proto-Indo-European*h₁én
2. Proto-Indo-European*h₁n̥tér
3. Proto-Italic*n̥ter
4. Latininter
5. Latininter-
6. Englishinter-
7. Proto-Indo-European*ḱe?
8. Proto-Indo-European*ḱóm
9. Proto-Italic*kom
10. Proto-Italic*kom-
11. Latincom-
12. Proto-Indo-European*perh₃-
13. Proto-Indo-European*pérh₃tis?
14. Proto-Italic*partis
15. Latinpars
16. Latinpartiō
17. Latinpartior
18. Latincompartīrī
19. Italiancompartire
20. Italiancompartimento
21. Middle Frenchcompartiment
22. Englishcompartment
23. Proto-Indo-European*h₂el-
24. Proto-Indo-European*h₂élisder.?
25. Englishcompartmental
26. English todayintercompartmental

From inter- + compartmental.