The quality or state of being so simple or well-designed that even a foolish person cannot make it fail or use it incorrectly.
From 'foolproof' (fool + proof, meaning protected against) combined with '-ness' (Old English suffix forming abstract nouns), creating a noun describing the state of fool-proofing.
The term 'foolproof' became popular during the 20th century as technology and manufacturing demanded designs so intuitive that user error was impossible—it's a fascinating window into how industrial design shaped our vocabulary.
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