Deciding to take what someone says ‘with a grain of salt’ means that you are skeptical about what you’re being told.
This idiom begins with the idea that something can be made more palatable with a pinch of salt. If somebody tells you something that’s difficult to believe, it’s something better to just swallow it than trying to dispute it.
Around 77AD lived Pliny the Elder, a writer and philosopher of the Roman Empire. He was responsible for translating an ancient recipe that was supposed to cure poisoning.
In his translation in Naturalis Historia, he wrote:
“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.”
The suggestion is that the ill effects of the concoction could be offset by a grain of salt.
While this is the first recorded use of the idiom, it became part of the common vernacular around the 17th century. In 1647 Christian Bible commentator, John Trapp wrote the Commentary on the Old and New Testaments and used the phrase: “This is to be taken with a grain of salt.”
Note: to take ‘with a pinch of salt’ is a later variant on the phrase and more widely used in English and Australian vernacular. Its earliest use appears to be in F. R. Cowell’s 1948 work, titled Cicero & the Roman Republic:
“A more critical spirit slowly developed, so that Cicero and his friends took more than the proverbial pinch of salt before swallowing everything written by these earlier authors.”