unthinkable
adj. B2 Upper Intermediate US //ənˈθɪŋkəbəɫ// UK //ʌnθˈɪŋkəbəl// un·think·able
adj. impossible to imagine or accept because it is so shocking or unlikely. You use this when something is too extreme to even consider.
adj. impossible to imagine or consider as a possibility; often used to describe events or ideas that are socially, morally, or practically unacceptable.
A few years ago, a global lockdown seemed unthinkable.
The loss of the company's main server would have been unthinkable before they implemented the new backup system.
In the current political climate, a compromise that was once considered standard has become unthinkable to both parties, leading to a total stalemate in negotiations.
Usage
Often used predicatively after linking verbs like 'be', 'seem', or 'become'.