The phrase "take [something] with a grain of salt" refers to an Ancient Roman recipe for an antidote that protects against poisons.
Today, taking something with a grain of salt, means approaching it with a certain degree of skepticism; not believing everything you hear or see. But in 77 BC, when the phrase first appeared in Pliny’s book Historia Naturalis, it was just part of a recipe! According to the book, adding a grain of salt to two dried walnuts, two figs, and twenty leaves of rue creates an antidote for poison.
However, a person must take this poison while fasting in order to be immune from all poisons for the rest of the day. The use of the phrase in English dates to 1647 when it appeared in John Trapp’s Commentary on Revelation.
Both the modern and the ancient Latin meanings of the phrase have to do with the idea of guarding one’s self from the harmful effects of something.