In*ef`fi*ca"cious (?), a. [Pref. in- not + efficacious: cf. F. inefficace, L. inefficax.]
Not efficacious; not having power to produce the effect desired; inadequate; incompetent; inefficient; impotent.
The authority of Parliament must become inefficacious . . . to restrain the growth of disorders.
⇒ Ineffectual, says Johnson, rather denotes an actual failure, and inefficacious an 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.
© Webster 1913.