The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

powerless
    adj 1: lacking power [ant: powerful]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "powerless":
   anemic, asthenic, bloodless, chicken, cowardly, crippled,
   debilitated, decrepit, disabled, drooping, droopy, dull, effete,
   etiolated, faint, faintish, featherweight, feeble, flabby, flaccid,
   floppy, forceless, gone, gutless, helpless, imbecile, impotent,
   inactive, incapable, incapacitated, incompetent, ineffective,
   ineffectual, inefficacious, inert, infirm, languid, languorous,
   lightweight, limber, limp, listless, lustless, marrowless,
   nerveless, no-account, of no account, passive, pithless, pooped,
   rubbery, sapless, sinewless, slack, soft, spineless, strengthless,
   supine, unable, unauthoritative, unfit, unhardened, unnerved,
   unstrung, weak, weakly, without any weight