absurd

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

• adjective Extremely unreasonable, incongruous, or inappropriate.
• adjective Impossible to take seriously; silly: synonym: foolish.
• adjective Of, relating to, or manifesting the view that there is no order or meaning in human life or in the universe.
• adjective Of or relating to absurdism.
• noun The condition or state in which humans exist in an absurd universe, without meaning or purpose. Used chiefly with the.

from The Century Dictionary.

• Being or acting contrary to common sense or sound judgment; inconsistent with common sense; ridiculous; nonsensical: as, an absurd statement; absurd conduct; an absurd fellow.
• Specifically In logic or philosophy, inconsistent with reason; logically contradictory; impossible: as, that the whole is less than the sum of its parts is an absurd proposition; an absurd hypothesis.
• Synonyms Absurd, Silly, Foolish, Stupid, Irrational, Unreasonable, Preposterous, Infatuated, ridiculous, nonsensical, senseless, incongruous, unwise, ill-judged, ill-advised. (See foolish.) Foolish, absurd, and preposterous imply a contradiction of common sense, rising in degree from foolish, which is commonly applied where the contradiction is small or trivial. That which is foolish is characterized by weakness of mind, and provokes our contempt. That which is silly is still weaker, and more contemptible in its lack of sense; silly is the extreme in that direction. That which is absurd does not directly suggest weakness of mind, but it is glaringly opposed to common sense and reason: as, that a thing should be unequal to itself is absurd. That which is preposterous is the height of absurdity, an absurdity as conspicuous as getting a thing wrong side before; it excites amazement that any one should be capable of such an extreme of foolishness. That which is irrational is contrary to reason, but not especially to common sense. Unreasonable is more often used of the relation of men to each other; it implies less discredit to the understanding, but more to the will, indicating an unwillingness to conform to reason. Irrational ideas, conclusions; unreasonable demands, assumptions, people. An infatuated person is so possessed by a misleading idea or passion that his thoughts and conduct are controlled by it and turned into folly. He who is stupid appears to have little intelligence; that which is stupid is that which would be natural in a person whose powers of reasoning are defective or suspended.
• noun An unreasonable person or thing; one who or that which is characterized by unreasonableness; an absurdity.

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

• noun obsolete An absurdity.
• adjective Contrary to reason or propriety; obviously and flatly opposed to manifest truth; inconsistent with the plain dictates of common sense; logically contradictory; nonsensical; ridiculous

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

• adjective Having no rational or orderly relationship to people's lives; meaningless; lacking order or value.
• adjective Dealing with absurdism.

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

• adjective incongruous;inviting ridicule
• adjective inconsistent with reason or logic or common sense
• noun a situation in which life seems irrational and meaningless