foolproof

[ fool-proof ]
/ ˈfulˌpruf /

adjective

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

Origin of foolproof

An Americanism dating back to 1900–05; fool1 + -proof
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for foolproof

British Dictionary definitions for foolproof

foolproof

/ (ˈfuːlˌpruːf) /

adjective

proof against failure; infalliblea foolproof idea
(esp of machines) proof against human misuse, error, etc
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012