Source : Webster's Revised Unabridged Dictionary (1913)

Inefficaciousness \In*ef`fi*ca"cious*ness\, n.
   Want of effect, or of power to produce the effect;
   inefficacy.

Source : WordNet®

inefficaciousness
     n : a lack of efficacy [syn: {inefficacy}] [ant: {efficacy}]