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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. [ 1913 Webster ]

The authority of Parliament must become inefficacious . . . to restrain the growth of disorders. Burke. [ 1913 Webster ]

☞ 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. [ 1913 Webster ]