unthinkable
un-think-a-ble
adjective
Too extreme or dreadful to be considered or imagined as possible.
"Such a betrayal was simply unthinkable."
noun
An event or situation too dreadful or impossible to contemplate.
"They had to prepare for the unthinkable."
Thesaurus
Synonyms: out of the question, inconceivable, impossible, unimaginable
Antonyms: thinkable