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un‧think‧a‧ble

1 impossible to accept or imagine :
it would be unthinkable for somebody to do something
It would be unthinkable for me to stay anywhere but with the family.
It would be unthinkable for me to stay anywhere but with the family.2 something that is impossible to accept or imagine :
It was the job of the committee to think the unthinkable (=plan for unexpected events or situations).
the unthinkable
It was the job of the committee to think the unthinkable (=plan for unexpected events or situations).