If you have a freezing day, a powerful desire to get drunk as quickly as possible, and a bunch of beer, here's a way to increase the concentration of alcohol in your brew. Learn about jacking. Yes, it's called that.
In the old days, people were even more interested in getting drunk than we are now, but had less legal resources to do it. Many places had restrictions on the percentage of alcohol that could be sold in drinks, and sometimes home brewers were not able to get the alcohol content they liked on the first attempt. Distillation could be tough. Done wrong, it could burn off the alcohol without evaporating the water, thereby diluting the alcohol instead of intensifying it. On the other hand, on cold winter nights, the brewers could always jack their product.
Jacking was the common term for what's now known as fractional freezing. It relies on the fact that alcohol doesn't freeze as easily as water. Freezing beer, wine, or apple cider could up the alcohol content relatively easily. (This is the process for which applejack is named.) The main problem with the process was that it lost people some alcohol. Fractional freezing starts below 0 degrees, and what freezes first isn't pure water – it's water laced with a tiny bit of alcohol. Still, the proportion of alcohol in the ice is much, much lower than the proportion of the alcohol in the overall drink, so letting it partially freeze, then fishing out the ice crystals, is an easy way to make a harder drink.
The process is still occasionally used by beer, cider, and even wine makers.