The mundane meaning is simply molasses –– which is the residual product of the sugar refining process. But the word has an older and more romantic meaning: an antidote for the bite of wild beasts. Venice treacle, for example, was a compound of 64 drugs in honey –– which sounds like the electuary mentioned in The Yeomen of the Guard. See Brewer (54). Paulshock (238) avers that the best treacles always included “chopped, powdered, or distilled vipers, as well as opium, honey, and a slew of other ingredients, many very rare and difficult to obtain.” She adds that “the original recipe called for periods of storage for up to ten years to allow all 50 or so ingredients to ferment and blend … ” I think that is all very interesting; but I also think Buttercup was selling molasses, or something like it, and not that vile medicine.