The word 'treacle' has a fascinating journey from poison cure to sugary syrup. It entered English via Old French 'triacle', from Latin 'theriaca', itself borrowed from Greek 'thēriakē', a term for a broad-spectrum antidote, especially one involving viper's flesh. This medicinal sense persisted in English for centuries, referring to a healing compound or salve. The semantic shift occurred because such ancient antidotes were often prepared with a thick, honey-like base. Over time, the association with the sweet, viscous vehicle became dominant, leading to the modern meaning of a thick molasses, and later, metaphorically, to overly sweet sentiment.

The word is a simplex form. It originates from the Latin 'theriaca', which came from the Greek 'thēriakē', meaning "antidote against venomous bites".