Foolproof (adjective)

1. So simple and straightforward that it cannot be misunderstood or fail.
2. Designed to prevent mistakes or misuse.

Origin:

Early 20th century: from fool + proof.

Examples:

1. The recipe is foolproof and easy to follow.
2. The design of the device is foolproof, making it difficult to use incorrectly.
3. The instructions were foolproof, so even a child could understand them.
4. The plan was foolproof and couldn't fail.
5. The system was designed to be foolproof to prevent human error.