/ Dictionary / Index I inefficacy: Meaning and Definition of Find definitions for: in•ef•fi•ca•cy Pronunciation: (in-ef'i-ku-sē), [key] — n. lack of power or capacity to produce the desired effect. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: inefficacy (Thesaurus) inefficacious inefficiency Related Content Daily Word Quiz: spectral Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language