Something strange has happened to the useful expression “foolproof.” Many writers are writing “full proof” to mean “safe against misinterpretation, misuse, or failure”:
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The expression foolproof originated in 1902 as an Americanism meaning “safe against the incompetence of a fool.” It combines the words fool and proof.
fool: a person lacking in judgment or prudence; a person who acts stupidly or recklessly
proof: The evidence or argument that compels the mind to accept an assertion as true.
Foolproof follows the pattern of such words as fireproof and waterproof and means that something has been tested and proved to withstand certain damaging agents.
The growing use of the expression “full proof” in the sense of “foolproof” may stem from a reluctance to cause offense to the fool demographic.
Yet the definition in Merriam-Webster Unabridged skillfully manages to define foolproof without the slightest mention of the wisdom-challenged portion of the population:
1 : so simple, plain, or strong as not to be liable to be misunderstood, damaged, or misused
2 : guaranteed to operate without breakdown or failure under any conditions
There probably are contexts in which the expression “full proof” can be justified. For example, one might demand “full proof” of identity. Although it seems to me that “proof” would suffice.
The expression “to make full proof” occurs in the King James translation of the Bible:
But watch thou in all things, endure afflictions, do the work of an evangelist, make full proof of thy ministry. 2 Timothy 4:5
The expression and discussions of its meaning are to be found on many evangelical sites:
What constitutes “full proof” in Paul’s advice to Timothy?
I think “make full proof of” means to fulfill the ministry that God gives you to do.
If you just can’t bear to use an expression that you fear may suggest you’re calling someone a fool, here are a few words you could substitute in certain contexts: