E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
properly means an antidote against the bite of wild beasts (Greek, theriaka [pharmka], from thr a wild beast). The ancients gave the name to several sorts of antidotes, but ultimately it was applied chiefly to Venice treacle (theriaca androchi), a compound of some sixty-four drugs in honey.
Sir Thomas More speaks of a most strong treacle (i.e. antidote) against these venomous heresies. And in an old version of Jeremiah viii. 22, balm is translated treacleIs there no treacle at Gilead? Is there no phisitian there?