Definition of foolproof adjective from the Oxford Advanced Learner's Dictionary

 

foolproof

 adjective
adjective
BrE BrE//ˈfuːlpruːf//
 
; NAmE NAmE//ˈfuːlpruːf//
 
 
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(of a plan, machine, method, etc.) very well designed and easy to use so that it cannot fail and you cannot use it wrongly synonym infallible This recipe is foolproof—it works every time. No system can ever be completely foolproof. The system is virtually foolproof. We’re trying to work out a way to make the system foolproof.
See the Oxford Advanced American Dictionary entry: foolproof