The term "foolproof" originates in 1902. The term "idiot-proof" became popular in the 1970s. It may have been invented as a stronger-sounding version of foolproof, as the force of foolproof had declined due to frequent usage.

Is it fail proof or foolproof?

The correct term to use in this instance is foolproof. It is sometimes hyphenated as fool-proof, but the conventional and more widely accepted spelling is as a single word. Used as an adjective, foolproof means that something is infallible. It is incapable of going wrong.

What is foolproof method?

a foolproof method, plan, or system is so well designed that it cannot go wrong or is certain to succeed. He devised what he thought was a foolproof plan. Synonyms and related words.