undividable

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*dwóh₁
9. Proto-Indo-European*trísinflu.
10. Proto-Indo-European*d(w)is-
11. Proto-Italic*dis-
12. Latindis-
13. Proto-Indo-European*dwóh₁
14. Proto-Indo-European*dwi-
15. Proto-Indo-European*dʰeh₁-
16. Proto-Indo-European*h₁weydʰh₁-
17. Proto-Italic*wiðō
18. Latin*vidō
19. Latindīvidō
20. Middle Englishdividen
21. Englishdivide
22. Englishdividable
23. English todayundividable

From un- + dividable.