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inefficacy

[in-ef-i-kuh-see]
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noun
  1. lack of power or capacity to produce the desired effect.
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Origin of inefficacy

From the Late Latin word inefficācia, dating back to 1605–15. See in-3, efficacy
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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Word Origin and History for inefficacy

n.

"want of force or virtue to produce the desired effect," 1610s, from Late Latin inefficacia, from inefficacem (nominative inefficax), from in- "not, opposite of" (see in- (1)) + efficax (see efficacy).

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Online Etymology Dictionary, © 2010 Douglas Harper