Impotent means lacking power or authority. Captain Bligh was impotent in the face of Christian's mutiny. It also refers to a man's inability to achieve or maintain an erection. If a man is sterile, or unable to reproduce, he is also called impotent.
Those wacky ancients really knew how to mess with you — simply add the prefix im to the front of potent, which means "powerful," and you get its opposite — "powerless." While Viagra can help when the problems are physiological, an impotent leader has lost his ability to govern. No little blue pill will dig him out of that hole. It can also allude to a certain meaninglessness, such as the music critic who wrote that the easy listening genre of pop was so dull as to be "the soundtrack to impotence."