Etymologically
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efficacy

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Pronunciation

  • /ˈɛf.ɪ.kə.si/[1]

Etymology

It is borrowed from Latin efficācia (“efficacy, efficiency”),[2][3][4][5][6] from efficāx (“powerful, effectual”),[3][4][5] from efficiō (“to work out, accomplish”),[3][4][7] the source of effect.[3][4] It replaced efficace, probably first recorded before 1200 in the “Ancrene Riwle”, from Old French efficace,[4][3] and efficacite, from the early 15th century, from Latin efficācitās.[3][4] The Anglo-Norman noun is recorded from 1160–74, for efficacy, virtue and power to produce an effect.[8]

Earliest recorded meaning

The power to produce an effect, or the intended effect: 1527, in L. Andrewe's translation of Brunschwig's Vertuose Boke.[2][4][3] Merriam-Webster Unabridged dates the word to the 13th century.[9]

Semantic evolution

The main sense has not changed since the 16th century.[2][6] For a time the word also meant an effect (from 1549, in Latimer, until 1614),[2][6] and Locke used it in 1690 for a way of bringing about a result.[2] Johnson (1755) defined it as the power to produce effects.[10]

Timeline

  • before 1200 – the earlier efficace.[4][3]
  • early 15th century – the earlier efficacite.[3]
  • 1527 – power to produce effects.[2][4]
  • 1549 – an effect (obsolete by 1614).[2]
  • 1690 – a way of bringing about a result, in Locke.[2]

Meanings

Noun

  1. The power or ability to produce the intended result; effectiveness: They doubted the efficacy of the new treatment.[9][11][2][6]
  2. (obsolete, 1549–1614) An effect.[2][6]

Word family

  • inefficacy
  • nonefficacy

Usage notes

The OED notes that efficacy is not used of people as agents, unlike efficacious.[2] Collins gives efficaciousness as a synonym.[11]

References

  1. ^ Wiktionary, the free dictionary, https://en.wiktionary.org
  2. ^ a b c d e f g h i j k Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
  3. ^ a b c d e f g h i Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
  4. ^ a b c d e f g h i Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
  5. ^ a b The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
  6. ^ a b c d e Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
  7. ^ Skeat, Walter W. (1911), A Concise Etymological Dictionary of the English Language, new ed., Oxford: Clarendon Press
  8. ^ Anglo-Norman Dictionary (AND2), Aberystwyth University, anglo-norman.net (2nd ed., 2005–)
  9. ^ a b Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
  10. ^ Johnson, Samuel (1755), A Dictionary of the English Language, London; 4th ed. 1773; Johnson's Dictionary Online, ed. Beth Rapp Young et al.
  11. ^ a b Collins English Dictionary (2023), Glasgow: HarperCollins