Powerless, 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.
Shades of meaning
Fernald lists powerless only as an antonym, under formidable.
He lists it without comment; the entries above explain its neighbors.
Synonyms
none given