The convention of measuring the concentration of alcohol in “proof” originated in the British Army, where they would test the potency of their beverage by pouring it on a small pile of gunpowder. If the gunpowder would still ignite, it was “proof” that there was a proper concentration of alcohol. The threshold amount of alcohol that is necessary for this is actually a bit more than 50% (57%, I think), but the U.S. standardized some time ago on 50% alcohol being 100 proof. (The Brits use a system with the above-mentioned 57% being 100 proof. More accuracy, but more math. There is undoubtedly some conclusion to be drawn here about the fundamental differences between Americans and Brits, but I have homework to do.)
Courtesy of my rocking Psychopharmacology text.