Designed so that even someone acting foolishly cannot cause it to fail or malfunction; extremely safe from error.
Compound of 'folly' (foolishness) plus 'proof' (resistant to or protected from). Modeled on compounds like 'foolproof' and 'waterproof'.
Though 'foolproof' is the standard term, 'follyproof' occasionally appears in literature—it's a reminder that English has built-in synonym flexibility where the same concept can be expressed slightly different ways.
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