<p><strong>Adjective</strong></p><ul><li>inconceivable; unimaginable: the unthinkable size of the universe</li><li>not to be considered; out of the question: Such a suggestion is unthinkable</li></ul><p><strong>Noun</strong></p><ul><li>something that cannot be conceived or imagined, as something too unusual, vague, or disagreeable: Today's unthinkables are tomorrow's realities</li></ul>