Grain of salt
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 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 (small amount) of wit", actually to "with caution"/cautiously. 
The phrase is said "with a pinch of salt" in British English and said "with a grain of salt" in American English.
- The dictionary definition of grain of salt at Wiktionary