unthinkably

/ʌnˈθɪŋkəbli/
adverb
  1. In a way that is impossible to imagine or accept; to an extreme degree.
    • She behaved unthinkably, leaving her friends in danger without a second thought.
    • The cost of the new stadium was unthinkably high.
    • The temperature dropped unthinkably low, breaking all records.
Antonyms