Pronunciation : In*ef`fi*ca"cious Part of Speech : a. Etymology : [Pref. in- not + efficacious: cf. F. inefficace, L. inefficax.] Definition : Defn: Not efficacious; not having power to produce the effect desired; inadequate; incompetent; inefficient; impotent. Boyle. The authority of Parliament must become inefficacious . . . to restrain the growth of disorders. Burke.
Note: Ineffectual, says Johnson, rather denotes an actual failure, and inefficacious and habitual impotence to any effect. But the distinction is not always observed, nor can it be; for we can not always know whether means are inefficacious till experiment has proved them ineffectual. Inefficacious is therefore sometimes synonymous with ineffectual. Source : Webster's Unabridged Dictionary, 1913