Answer: Gunpowder

The history of sailors as sea-hardened and hard-drinking is certainly a well earned history and there were more than a few ways a good captain would never want to cross his crew. Among those things was watering down their daily ration of rum (the ration of which was considered directly part of the sailor’s wages).

In order to prove to the sailors that the barrels of rum and the servings therefrom had not been watered down to lower the alcohol content, the rum was “proofed” by soaking a pile of gunpowder with a serving of the rum.

If the gunpowder would still ignite after the dousing, that indicated the alcohol content of the rum was at or above 57.15% alcohol by volume (ABV) and was considered “at proof” or possibly (and even better as far as the sailors were concerned) “above proof.” To fail the test would mean the rum was “under proof” and would most certainly raise the ire of the crew. Overtime, as the science of distilling and chemistry improved, the gunpowder proof-test was replaced with a specific gravity test that measured the alcohol composition of the liquor in question.

Although alcohol is no longer proofed in such flash-bang fashion and most countries stick to labeling alcohol with the percent of alcohol in the form of X% ABV, you’ll still see bottles of liquor that have both the % label and terms like “proof.” Although the term no longer has a legal meaning, it remains in use as an indicator that the liquor is strong and in a nod to the relationship to the term “proof” in the first place.