Able to be easily deceived, tricked, or made to believe something untrue.
Fool (from Old English 'fōl', from Latin 'follis' meaning bellows or empty bag) plus the suffix '-able' (from Latin 'abilis'). The compound emerged naturally as English speakers created adjectives describing who could be fooled.
Foolable is interesting because it's used strategically in psychology and marketing—researchers study how 'foolable' human brains are to optical illusions and false information, revealing that even smart people have predictable blind spots.
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