Navy Strength Gin
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Navy Strength Gin was thus called because it was bottled at an alcoholic strength that would still ignite when mixed with gunpowder. That alcohol by volume strength was around 58%.
The British Navy used this test to ensure that their suppliers weren't cheating them by supplying Gin that was less than the agreed alcoholic strength.
If it ignited, it was proof that the gin hadn't been diluted.
Thus, the word "proof".