Grain of salt
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(With) a grain of salt is a literal translation of a Latin phrase, (cum) grano salis.
In common parlance, if something is to be taken with a grain of salt, it means that a copious measure of skepticism should be applied regarding a claim; that it should not be blindly accepted and believed without any doubt or reservation. According to the Oxford English Dictionary "to take 'it' with a grain of salt" means "to accept a thing less than fully". It dates this usage back to 1647.
The phrase comes from 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.