noneffective

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*h₁éǵʰ
15. Proto-Indo-European*h₁éǵʰs
16. Proto-Italic*eks
17. Latinex
18. Latinex-
19. Proto-Indo-European*dʰeh₁-
20. Proto-Indo-European*dʰeh₁k-
21. Proto-Indo-European*dʰh₁kyéti
22. Proto-Italic*θakjō
23. Proto-Italic*fakjō
24. Latinfaciō
25. Latinefficiō
26. Latineffectīvu
27. Frencheffectif
28. Englisheffective
29. English todaynoneffective

From non- + effective.