Of a device: protected against, or designed to be proof against, misuse or error.
2
Of an idea or plan: certain to succeed in all eventualities, or claimed to be so; infallible.
Dill had hit upon a foolproof plan to make Boo Radley come out at no cost to ourselves (place a trail of lemon drops from the back door to the front yard and he’d follow it, like an ant).1960 July 11, Harper Lee, chapter 15, in To Kill a Mockingbird, Philadelphia, Pa.; New York, N.Y.: J[oshua] B[allinger] Lippincott Company, →OCLC:
My foolproof recipe is my gâteau fermière.2021 June 1, Marie-Dominique, “‘Throw the ingredients together with reckless abandon!’: Guardian readers on their tastiest foolproof bakes”, in The Guardian, archived from the original on 03 Jul 2021:
1 more example
From Nigella Lawson’s effortless spaghetti with Marmite to Chetna Makan’s foolproof chicken curry – the recipes to turn to when you want easy but tasty suppers2022 January 24, Allan Jenkins, Molly Tait-Hyland, “The 20 best easy dinner recipes – every one simple and delicious”, in The Guardian, archived from the original on 09 Mar 2022: