Outside of the Carribean, rum is typically sold at a diluted strength of around 80 proof (40% alcohol by volume, or thereabouts). Overproof rum is the undiluted version, typically sold at about 160 to 190 proof.
In addition to providing a great kick in punches and other large-scale preparations, it’s also highly flammable – awesome! Obligatory safety warning: Treat overproof rum like a can of gasoline. Don’t smoke around it, don’t light a match… hell, don’t even twerk around it.
