The stuff is purchased from Honest Jimmy of the Trouble Brewing minigame on the pirate island of Mos Le'Harmless for 50 Pieces of eight. To access the minigame and shop (as well as the rest of the island), you must have completed Cabin Fever. "The stuff" apparently contains some questionable and/or toxic content, as it has been banned by the Cooking Guild of Varrock. It does not actually cause any negative side effects when brewing, however.
The stuff is used after placing two buckets of water and two handfuls of barley malt into the fermenting vat. The unfinished brew can then be followed by the ingredients for the desired ale and finished with a pot of ale yeast. The odds of the player brewing a mature ale increases from 5% to 64% if the stuff is added to the fermenting vat.
To brew you must go to a brewery, either the one found in eastern Keldagrim, or the one under the inn in Port Phasmatys.
- "The stuff" is most likely a reference to the 1985 movie of the same title.