The idea behind the expression originally comes from eating practices of antiquity – namely, ancient writers would often suggest that food is easier to swallow if taken with a small amount of salt.
However, one of these writers, Pliny the Elder, used the phrase not for merely adding flavour to a meal but as part of an antidote to poison. In his Naturalis Historia from AD 77, he mentions the method of preventing poisons once used by King Mithridates VI of Pontus:
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 salt may not have been a vital ingredient in the recipe, but as a way of making the mixture of walnuts, figs and rue easier to digest.
The more modern use of the phrase has been around since the 17th century, most likely thanks to classical scholars’ studies of ancient texts. By the mid-20th century, the ‘grain’ transformed into ‘pinch’, which remains the more commonly used term today.
The earliest known record of this new version of the phrase comes from 1948, and FR Cowell’s 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”. And thus, a new expression for the sceptics out there was born.