treacle (n.)
mid-14c., "medicinal compound, antidote for poison," from Old French triacle "antidote, cure for snake-bite" (c.1200), from Vulgar Latin *triacula, from Latin theriaca, from Greek theriake (antidotos) "antidote for poisonous wild animals," from fem. of theriakos "of a wild animal," from therion "wild animal," diminutive of ther (genitive theros) "wild animal," from PIE root *ghwer- "wild" (see fierce).

Sense of "molasses" is first recorded 1690s (the connection may be from the use of molasses as a laxative, or its use to disguise the bad taste of medicine); that of "anything too sweet or sentimental" is from 1771. Related: Treacly.