Etymology
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inefficacious (adj.)

"not producing the desired effect," 1650s, from in- (1) "not, opposite of" + efficacious. Related: Inefficaciously; inefficaciousness (1640s).

updated on November 18, 2015

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Definitions of inefficacious from WordNet

inefficacious (adj.)
lacking the power to produce a desired effect;
laws that are inefficacious in stopping crime
From wordnet.princeton.edu, not affiliated with etymonline.