impossible

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Incapable of having existence or of occurring.
2. adj. Not capable of being accomplished: an impossible goal.
3. adj. Unacceptable; intolerable: impossible behavior.
4. adj. Extremely difficult to deal with or tolerate: an impossible child; an impossible situation.

Century Dictionary and Cyclopedia

1. Not possible; non-existent or false by necessity. Beyond the strength or power of the agent.
2. In law, in a stricter sense, prevented only by the act of God or a public enemy. Whatever a person binds himself by contract to do, if not absurd, is not regarded as impossible in this sense, if it might be accomplished by human means, these obstacles only excepted; and his practical inability is not deemed to render performance impossible.
3. Excessively odd; not to have been imagined; such as would not have been thought possible: as, she is a most impossible person; he wears an impossible hat.
4. n. An impossibility.

Wiktionary

1. adj. Not possible, not able to be done.
2. adj. colloquial, of a person Very difficult to deal with.
3. n. an impossibility
4. n. uncountable (with definite article) that which seems impossible

GNU Webster's 1913

1. adj. Not possible; incapable of being done, of existing, etc.; unattainable in the nature of things, or by means at command; insuperably difficult under the circumstances; absurd or impracticable; not feasible.
2. n. obsolete An impossibility.

WordNet 3.0

1. n. something that cannot be done
2. adj. not capable of occurring or being accomplished or dealt with
3. adj. used of persons or their behavior
4. adj. totally unlikely