unthinkable
Definitions
Too unlikely or improbable to be considered as a possibility.
Too shocking, extreme, or offensive to be accepted or imagined.
Examples
It was unthinkable that such a peaceful town could be the site of a major crime.
The idea of living without electricity in the modern world is simply unthinkable to most people.
Synonyms
inconceivable unimaginable implausible impossible preposterous out of the question