Something so shocking, terrible, or impossible that you can barely imagine it happening or even consider it.
From English prefix 'un-' (not) plus 'thinkable' (able to be thought), created in the 17th century to describe things that seemed beyond the reach of human thought or reason.
The word 'unthinkable' became especially powerful in the 20th century to describe historical atrocities—when people called the Holocaust 'unthinkable,' they meant that human minds could barely process that such evil could happen, which is both a word about thinking and a comment on how trauma breaks our ability to think clearly.
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