Something that is so simple or well-designed that even someone without much skill or intelligence cannot fail at it.
Formed from the words 'fool' (a silly person) and 'proof' (protected against), combining around 1900. It originally meant resistant to foolish mistakes, implying that something was so safe or straightforward that even a fool couldn't ruin it.
The phrase reflects an interesting optimism from the industrial era—designers believed they could engineer things so well that human error became impossible. Today, software companies still chase this ideal when creating user interfaces, though any tech support worker will tell you true 'foolproof' design remains elusive!
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