Alcoholic proof
The "proof" system was introduced in the 18th century. The British claim that this term has a nautical background. Every evening, sailors would line up for their daily quota of grog/rum. This had to first be proved to be as strong as promised and not watered down. The spirit was tested with gunpowder: a mixture of water and alcohol proved itself when it could be poured on a small amount of gunpowder and the wet powder could still be lit. If the powder did not ignite, the mixture had too much water in it and the  grog was considered below proof.