with a grain of salt

This catchphrase is a translation from Pliny’s Historia Naturalis, written c. 77 A.D.. The Latin is often quoted as cum grano salis, but this is incorrect. Pliny actually wrote addito salis grano. The full passage, from Book 23, section 149, is:

In sancutariis Mithridatis, maximi regis, devicit Cn. Pompeius invenit in peculiari commentario ipsius manu conpositionem antidoti e II nucibus siccis, item ficis totidem et rutae foliis XX simul tritis, addito salis grano: ei, qui hoc ieiunus sumat, nullum venenum nociturum illo die.
(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.)

So to take something with a grain of salt is to guard against its noxious effects.

Use of the phrase in English dates to 1647 when it appears in John Trapp’s Commentary on Revelation of John:

This is to be taken with a grain of salt.

(Sources: Oxford English Dictionary, 2nd Edition; Yale Book of Quotations; Perseus Digital Library, Tufts University)

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