un•think•a•blePronunciation: (un-thing'ku-bul), [key]
inconceivable; unimaginable: the unthinkable size of the universe.
not to be considered; out of the question: Such a suggestion is unthinkable.
something that cannot be conceived or imagined, as something too unusual, vague, or disagreeable: Today's unthinkables are tomorrow's realities.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.