The word is foolproof, meaning impervious to fools. Just as a bulletproof vest makes one invulnerable to bullets, a foolproof plan makes something invulnerable to fools. Foolproof is one word, without a hyphen (though some publications do include the hyphen—fool-proof). Full-proof, a common misspelling, makes no logical sense.
Examples
These writers use foolproof well:
The public knows that no security system can be foolproof, but some acknowledgement of accountability is warranted nonetheless. [Milwaukee Journal-Sentinel]
Apparently, DNA testing was seen as so foolproof that alibis didn’t count. [Herald Sun]
It can be tweaked according to the talent at hand, but nothing is foolproof, especially with young players. [Sports Illustrated]
Here, he shows a simple roasting technique that’s both foolproof and infinitely adaptable. [Globe and Mail]
The nonsensical full-proof is not nearly as common as foolproof, but it does occasionally appear in some normally well-edited publications and websites—for example:
No wind-down plan can be full-proof. [The Atlantic]
This strategy is not full-proof. [Washington Post]
We find no justification for this spelling. Let us know if you have any ideas.
