FWIW it's foolproof not "fullproof". The correct analysis is that this is proof against fools, in the same way that e.g. a rainproof electrical box is proof against rain, or a bulletproof armoured vehicle is proof against bullets.
This is an example of an eggcorn, humans acquire language by exposure, you misunderstand a word or saying, you analyse its apparent meaning based on what you thought you understood, and then you apply that analysis, it's a completely normal part of human language use. In some cases, eggcorns become normalised enough that it's reasonable to say they're just a variant use, but in most cases they'd be regarded as errors, and so it's probably useful to know if you've picked up any eggcorns. https://en.wikipedia.org/wiki/Eggcorn
This is an example of an eggcorn, humans acquire language by exposure, you misunderstand a word or saying, you analyse its apparent meaning based on what you thought you understood, and then you apply that analysis, it's a completely normal part of human language use. In some cases, eggcorns become normalised enough that it's reasonable to say they're just a variant use, but in most cases they'd be regarded as errors, and so it's probably useful to know if you've picked up any eggcorns. https://en.wikipedia.org/wiki/Eggcorn