Backset is similar to Dunder in Rum and sometimes the two are interchanged by producers – typically however, it relates to the process of Sour Mashing in American Whisky. Backset is the acidic residue that is left in the bottom of the beer still. Instead of being treated like waste, the backset is taken and added to a new batch of mash to increase the acidity of the liquid that will be fermented.
The acidic environment it helps to foster is key to the success of the fermentation. From many American distilleries, backset is 15-30% of the volume of the fermenter.