unable

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

• adjective Lacking the necessary power, authority, or means; not able; incapable.
• adjective Lacking mental or physical capability or efficiency; incompetent.
• adjective Usage Problem Not susceptible to action or treatment.

from The Century Dictionary.

• Not able.
• Lacking in ability; incapable.
• Weak; helpless; useless.

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

• adjective Not able; not having sufficient strength, means, knowledge, skill, or the like; impotent; weak; helpless; incapable; -- now usually followed by an infinitive or an adverbial phrase.

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

• adjective Not able; lacking a certain ability.

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

• adjective (usually followed by `to') not having the necessary means or skill or know-how
• adjective lacking in power or forcefulness
• adjective (usually followed by `to') lacking necessary physical or mental ability