unconceptualizable

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*ḱe?
9. Proto-Indo-European*ḱóm
10. Proto-Italic*kom
11. Proto-Italic*kom-
12. Latincon-
13. Proto-Indo-European*kap-
14. Proto-Indo-European*kapyéti
15. Proto-Italic*kapjō
16. Latincapiō
17. Ancient Greekσῠλλᾰμβᾰ́νω (sŭllămbắnō)
18. Latinconcipiō
19. Latinconceptus
20. Proto-Indo-European*h₂el-
21. Proto-Indo-European*h₂élisder.?
22. Medieval Latinconceptuālis
23. Englishconceptual
24. Englishconceptualize
25. Englishconceptualizable
26. English todayunconceptualizable

From un- + conceptualizable.