To “take with a grain [pinch] of salt” is an idiom which tells you to be skeptical, or take care not to interpret something literally.
The expression has been found in print in English starting in the mid-1600s, though it is probably much older.
We inherited our modern English word “salt” from the Old English “sealt.” The Latin word salis means both “salt” and “wit,” so that the Latin phrase cum grano salis could be translated as both “with a grain of salt” and “with a grain (small amount) of wit.” However, modern scholars say there’s no evidence in Latin literature of writers using salt as a figurative expression of scepticism. The Latin tag cum grano salis is very likely to be medieval Latin.
“Taken with a grain of salt” often means that the listener should to take into consideration the source of the information as possibly unreliable or prone to exaggeration and therefore the information itself should be “seasoned” to adjust for that bias.
The simplest explanation for “with a grain of salt” is that the phrase likens accepting a statement with skepticism to making an iffy dish of food more palatable with a dash of salt. There’s no doubt that salt will make dull food more exciting, or a tall tale easier to swallow, by sprinkling a bit of salt on it.
One 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. Here, salt is not the antidote, but taken merely to assist in swallowing the poison, just as we salt food to make it easier to swallow.
Salt plays a vital role in human civilization. Salt is not only a necessity of life but was also a symbol of worth and value. The phrase “to eat salt with” someone meant to enjoy their hospitality and friendship. Salt was also a metaphor for that which makes life worthwhile and interesting.
Random House Unabridged Dictionary says that the adjective “salty” means “piquant, sharp, witty.” It also means “racy, or coarse,” as in salty humor. Thus, we often describe someone as being salty, or as using salty language.
I’m well known for using salty language, usually when I’ve stubbed my bare toe on something, or run out of bread when I’m hungry.