Where did this ubiquitous phrase come from? Usually it is used in conjunction with either disputable of downright dubious information but I can't think of how salt helps the situation. The only thing I can think of is that since salt had a higher value in ancient times, that maybe the speaker is trying to almost bribe the recipient ("here is some info and here's some salt for your trouble") but I am just theorizing, take it with a grain of salt.
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The phrase is likely derived from the Latin cum grano salis, which in turn was used by Pliny the Elder in his work Naturalis historia:
It's apparently from one of Pliny's books - the recipes for antidote to poison began with "a pinch of salt" so you have some bad news (which is poison) but you take it with the antidote (a pinch of salt)
The motto of the USA came from his salad dressing recipe - so anything is possible!
It comes from the Italian avere sale in zucca, which literally means "to have salt in [your] pumpkin," where pumpkin is figuratively used to mean head; using zucca ("pumpkin") to mean testa ("head") is also used in the phrase essere una zucca vuota ("to be an empty pumpkin").
In avere sale in zucca, sale ("salt") is used to mean "to have a little of intelligence," and with a grain of salt (in Latin, cum grano salis) refers to using intelligence to judge something.
Etymonline reports that "to take something with a grain of salt" is from 1640s, from Modern Latin cum grano salis. Wikipedia reports what I reported in this answer, and which is what my teacher of Italian literature taught us when I was frequenting our scuole superiori.