coeffective

The story

1. Proto-Indo-European*ḱe?
2. Proto-Indo-European*ḱóm
3. Proto-Italic*kom
4. Proto-Italic*kom-
5. Latincon-
6. Latinco-
7. Englishco-
8. Proto-Indo-European*h₁éǵʰ
9. Proto-Indo-European*h₁éǵʰs
10. Proto-Italic*eks
11. Latinex
12. Latinex-
13. Proto-Indo-European*dʰeh₁-
14. Proto-Indo-European*dʰeh₁k-
15. Proto-Indo-European*dʰh₁kyéti
16. Proto-Italic*θakjō
17. Proto-Italic*fakjō
18. Latinfaciō
19. Latinefficiō
20. Latineffectīvu
21. Frencheffectif
22. Englisheffective
23. English todaycoeffective

From co- + effective.