In early times, distilled alcohol was mixed with gunpowder and lit. If it did not burn this proved it was less than 50% alcohol in water. If it burned evenly it was proof that the mix of cooh to h2o was 50 % If it burned too brightly it had more than 50% alcohol.
Today we know (US?) 50 % equal to !00 proof.
Typical that the US and British systems are yet again different - to get % proof in Britain you multiply by 1.751. Hence Woods 100% proof rum is actually 57.1% abv. And interestingly, a 70% proof bottle of English spirit is the same as a 80% proof bottle of American liquor (40%).