impotent

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Lacking physical strength or vigor; weak.
• adjective Lacking in power, as to act effectively; helpless.
• adjective Incapable of sexual intercourse because of an inability to achieve or sustain an erection.
• adjective Obsolete Lacking self-restraint.

from The Century Dictionary.

• Not potent; lacking power, strength, or vigor, physical, intellectual, or moral; powerless; weak; feeble.
• Wholly lacking in sexual power: said of the male, and rarely of the female.
• Lacking the power of self-restraint; destitute of self-command; ungovernable.
• noun One who is feeble, infirm, or languishing under disease.
• noun A male without sexual power.

from the GNU version of the Collaborative International Dictionary of English.

• noun rare One who is impotent.
• adjective Not potent; wanting power, strength. or vigor. whether physical, intellectual, or moral; deficient in capacity; destitute of force; weak; feeble; infirm.
• adjective Wanting the power of self-restraint; incontrolled; ungovernable; violent.
• adjective (Med.) Wanting the power of procreation; unable to copulate; also, sometimes, sterile; barren

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Lacking physical strength or vigor; weak.
• adjective Lacking in power, as to act effectively; helpless
• adjective Incapable of sexual intercourse, often because of an inability to achieve or sustain an erection.
• adjective of a male Sterile.
• adjective obsolete Lacking self-restraint.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective (of a male) unable to copulate
• adjective lacking power or ability