The proof is in the armor
The origins of the suffix -proof, ‘impervious to, protected against’, lie in the manufacture and testing of armor in fifteenth century England. Just as today one can speak of a “cold-weather proven” vehicle or “rain-tested” PVC windows, armor proven successful against the weapons of the day was called armour of proof, later proof armour. As an 1802 military dictionary explains, it was
…armour hardened so as to resist the force of an arrow, a sword, or other weapons in use before the discovery of gunpowder.
Around 1440, the Scot Sir Gilbert Hay, describes in a translation of a French work the confidence a knight gains by wearing such armor:
…sa that the traist that he has in his gude armouris makis him hardy, thinkand that nane may dere him, quhat perile that ever he be in, for thai ar of prove.
This sense of proof was both transferred and expanded figuratively, so that someone or something proof (adj.) against something could not be damaged or harmed by it, as if wearing proof armor. Still today, proof can be used in the original sense, as in this South African military history newsletter from 2016:
The armour was proof against shrapnel and rifle rounds fired from a distance greater than 500m. 2016.
Shakespeare’s Proof and the Armor of God
Shakespeare uses proof in this meaning twice in Romeo and Juliet, in both instances maintaining the military metaphor of proof armor. In 1, i, Romeo describes Juliet as impervious to Cupid’s arrow:
Well, in that hit you miss: she'll not be hit
With Cupid's arrow; she hath Dian's wit;
And, in strong proof of chastity well arm'd,
From love's weak childish bow she lives unharm'd.
And in 2, ii, Romeo speaks to Juliet:
Alack, there lies more peril in thine eye
Than twenty of their swords: look thou but sweet,
And I am proof against their enmity.
If Shakespeare were not enough to promulgate military metaphors to describe virtues, then there was always Ephesians 6.11–18, where in an extended metaphor of “the armor of God,” there is the “belt of truth,” the “sword of faith,” and the “helmet of salvation.”
Armour of proof, or a soveraign antidote, against the contagion of evil company 1655.
His virtue was proof against corruption. 1743.
A race of men may be raised up and educated with such principles of knowledge and virtue as will make them proof against the seductions of those who flatter the popular ignorance and caprice. — The Gazette of the United States (New York City), 22 Aug. 1789.
Much consideration was certainly due to his wounded feelings, but he should recollect that his character was proof against any attack of the kind. 1823.
After the invention of gunpowder, the common people believed magic could make one proof o’shot or proof o’lead, that is, impervious to lead bullets or shot, so that in 1728 the Scottish poet Allan Ramsay could describe a virtuous heart “proof a-shot to Birth or Money.”
Fireproof, but not exactly
One of the first compounds using -proof is fireproof, but if one considers how many scientific advances would yet be required to make something resistant to fire, then a religious context in 1652 shouldn't be surprising:
The fire of Chriſt’s coming, and no other, was that Ignis purgatorius which ſome of thoſse firſt Fathers harped on: Namely, they ſuppoſed this Divine Fire ſhould ſtretch even to the ſouls of the dead; And that ſuch as had departed out of this life not fully purged of ſin by Repentence here, ſhould not be found Fire-proof at that day, but be refined cum mora et dolore before their Reſurrection.
Bulletproof, waterproof, foolproof
While unfortunate souls in the cleansing fires of purgatory are not likely the direct ancestors of less metaphysical concepts like waterproof or bulletproof, they do show how easily such compounds could be formed once scientists, tinkerers, and inventors came up with the proper methods. Yet beneath each new coinage is hidden the armor of a fifteenth century knight.
