unthinkable

UK: /ʌnˈθɪŋkəb^əl/ US: /ʌnˈθɪŋkəbəl/ ,(un thing′kə bəl)

WordReference Random House Learner's Dictionary of American English © 2025

un•think•a•ble /ʌnˈθɪŋkəbəl/   adj. 

1. so terrible as to be inconceivable:
   an unthinkable crime.
2. not to be considered;
   out of the question.

WordReference Random House Unabridged Dictionary of American English © 2025

un•think•a•ble  (un thing′kə bəl), adj. 

1. inconceivable;
   unimaginable:
   the unthinkable size of the universe.
2. not to be considered;
   out of the question:
   Such a suggestion is unthinkable.

n. 

3. something that cannot be conceived or imagined, as something too unusual, vague, or disagreeable:
   Today's unthinkables are tomorrow's realities.

• 1400–50; late Middle English; see un-¹, thinkable

un′think•a•bil′i•ty, un•think′a•ble•ness, n. 
un•think′a•bly, adv. 

Collins Concise English Dictionary © HarperCollins Publishers::

unthinkable / ʌnˈθɪŋkəb^əl/ adjective

1. not to be contemplated; out of the question
2. unimaginable; inconceivable
3. unreasonable; improbable

unˈthinkably adverb