There are various synonyms that can be used instead of "idiot-proof" to describe a system or product that is foolproof and can be easily used by anyone. Some of the synonyms include "foolproof", "fail-safe", "user-friendly", "idiot-resistant", "easy to use", "simple", and "uncomplicated". These terms indicate that the system or product has been designed in such a way that it can be operated effortlessly without requiring any technical knowledge or understanding. They provide an assurance that the product passes through all the expected quality checks and can perform its function error-free even in the hands of an inexperienced user.