\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.
--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.