To take something with a "grain of salt" or "pinch of salt" is an English idiom that suggests to view something, specifically claims that may be misleading or unverified, with skepticism or to not interpret something literally.
Hypotheses of the phrase's origin include Pliny the Elder's Naturalis Historia, regarding the discovery of a recipe for an antidote to a poison. In the antidote, one of the ingredients was a grain of salt. Threats involving the poison were thus to be taken "with a grain of salt", and therefore less seriously.
The phrase cum grano salis ("with a grain of salt") is not what Pliny wrote. It is constructed according to the grammar of modern European languages rather than Classical Latin. Pliny's actual words were addito salis grano ("after having added a grain of salt").
An alternative account says that the Roman general Pompey believed that he could make himself immune to poison by ingesting small amounts of various poisons, and he took this treatment with a grain of salt to help him swallow the poison. In this version, the salt is not the antidote. It was taken merely to assist in swallowing the poison.
The Latin word sal (salis is the genitive) means both "salt" and "wit", thus the Latin phrase cum grano salis could be translated to either "with a grain of salt" or "with a grain of wit", actually to "with caution"/cautiously.
- The dictionary definition of grain of salt at Wiktionary