“Proof,” as it’s used in regards to booze, harkens back to when traders would have to literally prove that their hooch was the real deal. According to the University of Cincinnati’s William B. Jensen, in 16th century England, traders would drench a pellet of gun powder in liquor to determine the spirit’s potency. “If it was still possible to ignite the wet gunpowder, the alcohol content of the liquor was rated above proof and it was taxed at a higher rate, and vice versa if the powder failed to ignite.”
While the term “proof” stuck, in America, the standard it refers to has nothing to do with gunpowder. Around 1848, 50% alcohol by volume was chosen as a baseline and 100 was used as its corresponding proof. Thus, the proof is double the ABV.