Source : Webster's Revised Unabridged Dictionary (1913)

Powerless \Pow"er*less\, a.
   Destitute of power, force, or energy; weak; impotent; not
   able to produce any effect. -- {Pow"er*less*ly}, adv. --
   {Pow"er*less*ness}, n.

Source : WordNet®

powerless
     adj : lacking power [syn: {weak}] [ant: {powerful}]