countereffective

The story

1. Proto-Indo-European*ḱe?
2. Proto-Indo-European*ḱóm
3. Proto-Italic*kom
4. Proto-Italic*kom-
5. Proto-Italic*komteros
6. Proto-Italic*komterād
7. Latincontrā
8. Old Frenchcontre-
9. Anglo-Normancountre-
10. Middle Englishcounter-
11. Englishcounter-
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 todaycountereffective

From counter- + effective.