Scumble was introduced to fans of Terry Pratchett in the novel "Mort" as the Discworld equivalent of scrumpy (which Terry is personally, if briefly, familiar with).
According to the book, it's produced by preparing cider the usual way, and then leaving it outside in a barrel as the cold winter weather sets in. As the water freezes, it floats to the top, where it's removed regularly, leaving the alcohol (which freezes at a lower temperature) behind. The end result is a drink so concentrated it's ordinarily served in shot glasses the size of a thimble.