(Latin for grain of salt)
The idea for grain of salt as a remedy first appeared in Pliny’s Naturalis Historia, 77 A.D. and it translates this way:
After the defeat of that mighty monarch, Mithridates, Gnaeus Pompeius found in his private cabinet a recipe for an antidote in his own handwriting; it was to the following effect: Take two dried walnuts, two figs, and twenty leaves of rue; pound them all together, with the addition of a grain of salt; if a person takes this mixture fasting, he will be proof against all poisons for that day.
It was much later that the term took on a warning of sorts for things said/heard.
You can read about it here.