extracompartmental

The story

1. Proto-Indo-European*h₁éǵʰ
2. Proto-Indo-European*h₁éǵʰs
3. Proto-Indo-European*h₁eǵʰsteros
4. Proto-Italic*eksteros
5. Latinexter
6. Latinextrā
7. Englishextra-
8. Proto-Indo-European*ḱe?
9. Proto-Indo-European*ḱóm
10. Proto-Italic*kom
11. Proto-Italic*kom-
12. Latincom-
13. Proto-Indo-European*perh₃-
14. Proto-Indo-European*pérh₃tis?
15. Proto-Italic*partis
16. Latinpars
17. Latinpartiō
18. Latinpartior
19. Latincompartīrī
20. Italiancompartire
21. Italiancompartimento
22. Middle Frenchcompartiment
23. Englishcompartment
24. Proto-Indo-European*h₂el-
25. Proto-Indo-European*h₂élisder.?
26. Englishcompartmental
27. English todayextracompartmental

From extra- + compartmental.