Treacle is the usual British name for what Americans prefer to call molasses (remember the treacle well in Lewis Carroll’s Alice’s Adventures in Wonderland). The apothecaries’ term triacle is an older form of the word, but it was originally used for something very different — an antidote to the venom of a snake or insect.
This word came through French and Latin from Greek theriake, meaning an antidote against a poisonous bite, which has its origin in therion, a wild beast. Herbal remedies against such bites often contained alkaloids and so tasted bitter — it was common to sweeten them with honey to make them easier to swallow. English apothecaries changed over to black treacle as the sweetener when it began to be available in the Middle Ages.
The word was often used in a sense not so very different to that of balm or salve, some fragrant ointment or preparation used to heal or soothe the skin. In fact, the question in the book of Jeremiah in the Bible: “Is there no balm in Gilead; is there no physician there?” was translated by Miles Coverdale in 1535 using triacle instead of balm. Even earlier, Chaucer uses triacle in the sense of a salve in his Canterbury Tales: “Christ, which that is to every harm triacle”.
By the end of the seventeenth century, triacle had come to mean the molasses itself rather than the herbal remedy; by then the vowel had became slurred, the word dropped from three syllables to two, and it was respelled in the modern form, treacle.