unaccumulable

The story

1. Proto-Indo-European*né
2. Proto-Indo-European*n̥-
3. Proto-Germanic*un-
4. Proto-West Germanic*un-
5. Old Englishun-
6. Middle Englishun-
7. Englishun-
8. Proto-Indo-European*h₂éd
9. Proto-Italic*ad
10. Proto-Italic*ad-
11. Latinad-
12. Proto-Indo-European*ḱewh₁-
13. Proto-Indo-European*ḱuh₁mósder.?
14. Latincumulus
15. Latincumulō
16. Latinaccumulō
17. Latinaccumulātus
18. Middle Englishaccumylaten
19. Englishaccumulate
20. Englishaccumulable
21. English todayunaccumulable

From un- + accumulable.