From Wiki-pedia: Regarding 100 proof Origins.....( for what Wiki-is worth anyhow)
"In the 18th century and until 1 January 1980, the United Kingdom measured alcohol content in terms of "proof spirit", which was defined as the most dilute spirit that would sustain combustion of gunpowder.[1] The term originated in the 18th century, when payments to British sailors included rations of rum. To ensure that the rum had not been watered down, it was "proved" by dousing gunpowder in it, then tested to see if the gunpowder would ignite. If it did not, then the rum contained too much water and was considered to be "under proof". Gunpowder would not burn in rum that contained less than approximately 57.15% ABV. Therefore, rum that contained this percentage of alcohol was defined to have "100 degrees proof".