‘The term originated in the 18th century, when payments to British sailors included rations of rum. To ensure that it had not been watered down, the rum was “proved” by dousing gunpowder in it, then testing to see if it would ignite. If the gunpowder did not burn, the rum contained too much water—and was considered to be “underproof.”
A “proven” sample of rum was defined to be 100 degrees proof. This was later found to occur at 57.15% ethanol.’
—more at Wikipedia
And then later on, the U.S. standardized on the Alcohol By Volume standard, where degrees of proof is twice the alcohol content percentage.