impotent

adj. powerless, weak, helpless; unable to sustain an erection, unable to perform sexually (of a male)

Erectile dysfunction (ED or (male) impotence) is a sexual dysfunction characterized by the inability to develop or maintain an erection of the penis. There are various underlying causes, such as cardiovascular leakage and diabetes, many of which are medically treatable.The causes of erectile dysfunction may be physiological or psychological. Physiologically, erection is a hydraulic mechanism based upon blood entering and being retained in the penis, and there are various ways in which this can be impeded, most of which are amenable to treatment. Psychological impotence is where erection or penetration fails due to thoughts or feelings (psychological reasons) rather than physical impossibility; this can often be helped. Notably in psychological impotence there is a very strong placebo effect.
See more at Wikipedia.org...

Im·po·tent adj. 1. Incapable of sexual intercourse, often because of an inability to achieve or sustain an erection. 2. Sterile. Used of males. [more]Impotent - Community and Resources

Adjective
1. lacking power or ability; "Technology without morality is barbarous; morality without technology is impotent"- Freeman J.Dyson; "felt impotent rage"
(antonym) potent, strong
(similar) ineffective, ineffectual, unable
(see-also) powerless, weak
(attribute) potency, effectiveness, strength
2. (of a male) unable to copulate
(antonym) potent, virile

adj. impotent, powerless, weak, helpless; (of a male) unable to sustain an erection, unable to perform sexually