The story
- Proto-Indo-European*h₁éǵʰ
- Proto-Indo-European*h₁éǵʰs
- Proto-Italic*eks
- Latinex
- Latinex-
- Proto-Indo-European*dʰeh₁-
- Proto-Indo-European*dʰeh₁k-
- Proto-Indo-European*dʰh₁kyéti
- Proto-Italic*θakjō
- Proto-Italic*fakjō
- Latinfaciō
- Latinefficiō
- Latinefficāx
- Proto-Indo-European*h₃ed-
- Proto-Indo-European*h₃édosder.?
- Old Frenchefficacieux
- English todayefficacious
From Old French efficacieux (whence -acious), from Latin efficāx (“efficacious”) + -ous, from efficere (“to effect, to accomplish”); see effect.
First known use, by meaning
- from 1520s Effective; possessing efficacy.
Etymology from Wiktionary, CC BY-SA 4.0.