Etymology
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efficacy (n.)

"quality of being effectual, producing the desired effect," 1520s, from Latin efficacia "efficacy, efficiency," from efficax (genitive efficacis) "powerful, effectual, efficient," from stem of efficere "work out, accomplish" (see effect (n.)).

Earlier in English in the same sense was efficace (c. 1200), from Old French eficace (14c.), from Latin efficacia; also efficacite (early 15c.), from Latin efficacitatem.

updated on October 29, 2022

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