foolproof

[fool-proof] /ˈfulˌpruf/

adjective

involving no risk or harm, even when tampered with.

never-failing:

a foolproof method.

Origin

1900-05, Americanism; fool¹ + -proof

Examples from the web for foolproof

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.
If all that math was foolproof, after all, no one would need test pilots.
Buyers can read reviews that others have written about each seller, which fosters trust, though it is far from foolproof.
For now, though, vigilance remains the only safeguard-albeit not a foolproof one.
It is not foolproof: not even the rich can buy off all hazards, and rich countries and individuals will make poor decisions.
Still, this is foolproof subject matter for a novelist of limited gifts.
But the process is not foolproof because markets can also be wrong.

British Dictionary definitions for foolproof

/ˈfuːlˌpruːf/

adjective

proof against failure; infallible: a foolproof idea

(esp of machines) proof against human misuse, error, etc

Word Origin and History for foolproof

adj.

also fool-proof, 1902, American English, "safe against the incompetence of a fool," from fool (n.) + proof.