Proof was a term used to describe the alcohol content of a drink - historically rum.   To make sure that spirits were not watered down between producer & consumer, a little could be tested easily by soaking gunpowder in it.   The weakest alcohol strength that still enables gunpowder to be lit is 100% proof (57% alcohol by volume).

In the UKⁱ the proof of a drink is alcohol by volume multiplied by 1.75.   So a standard 40% spirit is 70% proof.

In the USA the proof is alcohol by volume multiplied by 2.