From Discworld & Terry Pratchett Wiki

Scumble is made from apples. Well, mainly apples, which in no way guarantees a low alcohol content. Scumble is drunk in thimblefuls, is strong enough to be used for cleaning spoons, and should not be put in a metal container or allowed to come into contact with water. Someone who can drink a lot of scumble all at once without ill effects is probably not really human (see Mort). Authentic scumble is made in Lancre, in a secret still owned and operated by Nanny Ogg. Nanny welcomed King Verence II's decree of prohibition, as she reckoned it would leave her with no competitors.

She does have many small competitors, most using freeze-distillation like some Applejack. Other producers ferment pure apple juice and freeze high-proof popskull out of it, a notorious beverage down into the plains. Nanny Ogg will only list her ingredients as "apples, mostly"; her brandy manages to be much more potent than normal brandy. A well-ordered still can increase the proof and eliminate unpleasant by-products, and a few minor additions from a Witch's pharmacopoeia can work wonders. Nanny Ogg's prowess with alcoholic beverages is well-known -- as is her product, known everywhere that men fall over backwards into ditches as 'suicider'.

The best apples for making scumble are: Lancre Blackheart, the Golden Disagreeable, and the Green Billet.

Related Roundworld drinks : Calvados (strong apple brandy distilled in Normandy), Apple Brandy and Scrumpy (which have long been produced in Somerset), and Applejack (made in North America) and a flattering imitation by Ales by Mail Nanny Ogg's Scumble Cider.