'foul proof' is a correct and usable phrase in written English.
It is an adjective used to describe something that cannot be defeated or made ineffective by any kind of foul play or trickery. For example, you could say "The referee's decision was foul proof and did not change after further review."
The good news for the Knicks was that the new, improved foul-proof Zo turned out to be a fable from the land of Oz.
The report lists seven tips to help ensure journalists' sources' security including data encryption and face-to-face meetings to avoid leaving a digital trail, but cautions that no measure is foul-proof.
Mr. Qian's family, some Chinese reporters and residents of Zhaiqiao Village cite the photographs as proof of foul play and a sloppy cover-up.
A photograph purportedly showing his feet was widely circulated on the Internet, causing a stir among activists who cited it as proof of foul play.
Though there is no proof of foul play, the circumstances surrounding Mr. Karpenko's final hours -- he received no serious medical attention for 24 hours after he collapsed and entered a state medical center -- have caused his family to doubt that he died of natural causes.
Although there is no proof of foul play, some have theorized that Stalin ordered his death to keep him quiet.
However, they have yet to discover proof of foul play -- something that others noted in response to Murdoch's tweet..@rupertmurdoch getting a bit ahead of the story?
U.S. and European security officials said there was no proof of foul play and there could be other explanations for the use of stolen passports.
28 As he explained, in any such charge within Scotland, the law started from the presumption that the child had been born dead until proof of foul play was adduced, the onus probandi thus falling upon the prosecutor.
There have been no concrete clues suggesting foul play, and there is no positive proof that either man didn't wander off on his own.
