nonphenomenal

The story

1. Proto-Indo-European*né
2. Proto-Germanic*ne
3. Proto-Indo-European*ís?
4. Proto-Indo-European*h₁óynos
5. Proto-Germanic*ainaz
6. Proto-Germanic*nainaz
7. Proto-West Germanic*nain
8. Old Englishnān
9. Middle Englishnon
10. Old Englishnān
11. Old Englishnān-
12. Middle Englishnon-
13. Englishnon-
14. Proto-Indo-European*bʰeh₂-
15. Proto-Indo-European*bʰh₂nyéti
16. Proto-Hellenic*pʰáňňō
17. Ancient Greekφαίνω (phaínō)
18. Ancient Greekφαινόμενον (phainómenon)
19. Late Latinphaenomenon
20. Englishphenomenon
21. Proto-Indo-European*h₂el-
22. Proto-Indo-European*h₂élisder.?
23. Ancient Greekφαινόμενον (phainómenon)
24. Frenchphénomène
25. Frenchphénoména
26. Englishphenomenal
27. English todaynonphenomenal

From non- + phenomenal.