foolproof
v.describing a plan or method that is very difficult to fail. You use this when you want to say something is safe and reliable.
v.describing a plan, method, or system that is highly unlikely to fail or be defeated. Often used to describe security measures or simple instructions that are hard to mess up.
“The new security system is foolproof.”
“The recipe is foolproof, so even if you are a beginner, you will get great results.”
“The architect designed the building with a foolproof drainage system to prevent any water damage during the heavy seasonal rains.”
“When Céline Dion’s era as a foolproof hitmaker ended, she seemed weirdly liberated and more willing to take risks.”
“Keep in mind that screen-time limits aren't foolproof.”
From fool + -proof.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.