In the 18th century, so the story goes, officers in the Royal Navy were concerned about their gin (or rum) being watered down – either by unscrupulous merchants, or by ratings stealing the odd snifter and replacing it with water. Then they discovered that gunpowder that had been soaked in spirits would still light, as long as the spirit had an ABV content of 57% or higher. (The actual value is more like 57.15%.) From that day forward, they insisted on their spirits having an ABV content of at least 57%, so that they could test it on board ship.
This naturally became known as 'navy strength', and it also became known as '100 degrees proof'.
This story is related on Wikipedia, as well as on any number of websites selling navy strength spirits (such as this one, for example). So I'm fairly confident that it's true.
In the United States, 100 degrees proof equates to 50% ABV. So a spirit produced in the USA will have a slightly higher proof rating than a similar–strength one produced in the UK.
© Haydn Thompson 2019