According to the Merriam-Webster Dictionary, to take something with a grain of salt means to consider information with a "skeptical attitude," and to not blindly trust information.

Most sources agree the phrase was first penned in 77 C.E. by Pliny the Elder in his text “The Natural History,” mentioning that it’s the final ingredient in an antidote for poison.

Pliny got this idea from the Third Mithridatic War, where the victorious Roman general Pompey searched through the fallen Persian King Mithridates' possessions.

Pompey found an antidote that read, via Mental Floss, "Take two dried walnuts, two figs, and 20 leaves of rue; pound them all together, with the addition of a grain of salt."

Another version of the phrase’s origin story is of a Roman general (not Pompey) who consumed small bits of poison so it wouldn't affect him.

To give the poison some flavor, he added a grain of salt to each one. This version of events is not widely accepted due to a lack of evidence.