equieffective

The story

1. Proto-Italic*aikʷos?
2. Old Latinaiquos
3. Latinaequus
4. Latinaequi-
5. Englishequi-
6. Proto-Indo-European*h₁éǵʰ
7. Proto-Indo-European*h₁éǵʰs
8. Proto-Italic*eks
9. Latinex
10. Latinex-
11. Proto-Indo-European*dʰeh₁-
12. Proto-Indo-European*dʰeh₁k-
13. Proto-Indo-European*dʰh₁kyéti
14. Proto-Italic*θakjō
15. Proto-Italic*fakjō
16. Latinfaciō
17. Latinefficiō
18. Latineffectīvu
19. Frencheffectif
20. Englisheffective
21. English todayequieffective

From equi- + effective.