Applejack is “hard” (alcoholic) apple cider which has been made even harder.
To make Applejack, the person concentrates the hard cider by getting rid of some of the water, which in turn increases the alcohol content of what is left. To do this, s/he places hard cider somewhere where it will freeze, such as on the back step in December. The water freezes at the top as a layer of ice, while the alcohol below doesn’t. This works because water will freeze solid at 32F (0C), while alcohol will remain liquid until temperatures reach a very frosty -173F (-114C.)
When the water has frozen into a solid ice layer, leaving the liquid alcohol below it, the ice layer is removed and discarded, and voilà, Applejack. The drink ends up being somewhere from 30 to 40% alcohol.
This is not a generally recommended procedure or outcome. There are reports of legendary headaches. Survivors advise never to drink it undiluted.
Commercially-made Applejack can be purchased. The juice from apples is fermented, then distilled, to give it an alcohol content similar to brandy and whiskey. It is usually aged at least two years so that it can be sold in the brandy class. The French brandy, Calvados, which is made from apples, is like an Applejack. But while Calvados is pure and aims to capture the essence of the apple, oftentimes Applejack only contains 35% apple — the rest is topped up with neutral grain spirits.
Laird & Company is the oldest Applejack producing operation in America, beginning business in Scobeyville, New Jersey in 1780. They received a special dispensation to continue brewing during Prohibition to produce Applejack for “medicinal purposes”.