undereffective

The story

1. Proto-Indo-European*h₁én
2. Proto-Indo-European*h₁entér
3. Proto-Indo-European*h₂én
4. Proto-Indo-European*dʰeh₁-?
5. Proto-Indo-European*(H)n̥dʰí
6. Proto-Indo-European*(H)n̥dʰér
7. Proto-Germanic*under
8. Proto-West Germanic*undar
9. Old Englishunder-
10. Middle Englishunder-
11. Englishunder-
12. Proto-Indo-European*h₁éǵʰ
13. Proto-Indo-European*h₁éǵʰs
14. Proto-Italic*eks
15. Latinex
16. Latinex-
17. Proto-Indo-European*dʰeh₁-
18. Proto-Indo-European*dʰeh₁k-
19. Proto-Indo-European*dʰh₁kyéti
20. Proto-Italic*θakjō
21. Proto-Italic*fakjō
22. Latinfaciō
23. Latinefficiō
24. Latineffectīvu
25. Frencheffectif
26. Englisheffective
27. English todayundereffective

From under- + effective.