‘Proof’, in terms of alcohol, came from the English Navy. Sailors wanted 'proof’ that their grog wasn’t being diluted, so they figured out a test. If you pour a drink that is 50% alcohol on gunpowder, the gunpowder can still be ignited, so that would be 100% proof that the grog was up to par. That’s why something that is 50% alcohol by volume is 100 proof.
Pharm prof, giving a really awesome history lesson