impotent

Short definition from Wiktionary

Adjective

1. 1
   Lacking physical strength or vigor; weak
2. 2
   Lacking in power, as to act effectively; helpless
3. 3
   Incapable of sexual intercourse, often because of an inability to achieve or sustain an erection, having impotentia coeundi.
4. 4
   Sterile
5. 5
   Lacking self-restraint

Noun

1. 1
   A man who has erectile dysfunction
2. 2
   An impotent or powerless person