Foolproof is a word that is often used to describe a plan, method, or system that is guaranteed to work perfectly and cannot fail. It is commonly used to refer to something that is secure, reliable, and infallible. However, for the purpose of variety and to avoid repetition, different synonyms can be used in place of foolproof. Some other words that can be used to describe something that is foolproof include infallible, bulletproof, fail-safe, guaranteed, foolproof, reliable, perfect, impeccable, sure-fire, and air-tight. These synonyms all convey a similar idea of certainty or trustworthiness and can add depth to one's writing by avoiding the overuse of the term "foolproof".