Etymology

inefficacious

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Today it means: Incapable of having the intended consequence. Full definition →

English efficacious ← Old French efficacieux ← Proto-Indo-European *h₃édosder.?
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The story

  1. Proto-Indo-European*né
  2. Proto-Indo-European*n̥-
  3. Proto-Italic*n̥-
  4. Latinin-
  5. Middle Englishin-
  6. Englishin-
  7. Proto-Indo-European*h₁éǵʰ
  8. Proto-Indo-European*h₁éǵʰs
  9. Proto-Italic*eks
  10. Latinex
  11. Latinex-
  12. Proto-Indo-European*dʰeh₁-
  13. Proto-Indo-European*dʰeh₁k-
  14. Proto-Indo-European*dʰh₁kyéti
  15. Proto-Italic*θakjō
  16. Proto-Italic*fakjō
  17. Latinfaciō
  18. Latinefficiō
  19. Latinefficāx
  20. Proto-Indo-European*h₃ed-
  21. Proto-Indo-European*h₃édosder.?
  22. Old Frenchefficacieux
  23. Englishefficacious
  24. English todayinefficacious

From in- + efficacious.

Etymology from Wiktionary, CC BY-SA 4.0.