treacle

Treacle comes from Middle English triacle, arriving partly through Old French triacle and partly through Old English tyriaca, both tracing to Late Latin *triaca/*tiriaca, a late form of theriaca. That word goes back to Ancient Greek θηριακή (thēriakḗ), meaning 'antidote,' the feminine form of θηριακός, 'concerning venomous beasts,' from θήρ (thḗr), 'beast.'

The facts don't detail a specific meaning shift, but the shape of the word's history — from an antidote against beasts to today's syrupy sweetener — suggests it moved a long way from its venom-fighting roots to its familiar place on the breakfast table.

Treacle is a doublet of theriac, both descending from the same Late Latin *triaca/*tiriaca root, just shaped by different paths into English.