Not fair or sensible; not based on logic or good judgment.
From Old French 'resonable' meaning rational or logical, from Latin 'rationabilis.' The prefix 'un-' negates the word, meaning lacking reason or logic.
The word 'unreasonable' assumes humans are naturally 'reasonable' creatures, but psychology has shown we're actually quite irrational—we make decisions based on emotions, biases, and snap judgments. So calling someone 'unreasonable' might actually mean they're being normally human!
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