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
- Late Latinefficācia
- Old Frenchefficace
- English todayefficacy
From Old French efficace, from Late Latin efficācia (“efficacy”) (whence -acy), from efficāx (“efficacious”); see efficacious.
Etymology from Wiktionary, CC BY-SA 4.0.