involving no risk or harm, even when tampered with.
never-failing: a foolproof method.

1900–05, Americanism; fool1 + -proof Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
foolproof (ˈfuːlˌpruːf)
1.  proof against failure; infallible: a foolproof idea
2.  (esp of machines) proof against human misuse, error, etc

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1902, Amer.Eng., "safe against the incompetence of a fool," from fool (n.) + proof (q.v.).
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Though a prevalent ploy in nature, the fake-eye look isn't foolproof.
The only foolproof way to improve the public understanding of science across
  the board is through the educational system.
The backup and containment systems are hardly as robust and foolproof as the
  article makes them out to be.
These tools can be lifesavers, but, they aren't foolproof.
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