Typically a term that’s connected to Rum, ‘overproof’ can be attributed to spirits that are pretty much any strength above 50% ABV. It’s more directly connected to Rum and Gin due to the spirits’ connection with the Navy.
The term came as a result of the way alcohol was measured. Before the early 1800’s there was no accurate way to measure the strength of a spirit, so Pursers in the British Navy would test the alcoholic strength of the rum they were buying by adding a few drops to some grains of gunpowder. The mixture was lit and if it ignited easily it was “poof” that it was of sufficient strength, if it went up with a bang, it was overproof, it would flame blue (and most likely go boom!).
See Navy Strength