Origin of "Take this question with a grain of salt"?

Solution 1:

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:

After the defeat of that mighty monarch, Mithridates, Cneius Pompeius found in his private cabinet a recipe for an antidote in his own hand- writing ; it was to the following effect: Take two dried walnuts, two figs, and twenty leaves of rue ; pound them all together, with the addition of a grain of salt; if a person takes this mixture fasting, he will be proof against all poisons for that day.

Solution 2:

With a grain of salt is a 1600s direct translation from Modern Latin cum grano salis (Etymoline), and salis is genitive of sal, which, in addition to ‘salt’, figuratively means ‘intellectual acuteness, good sense, shrewdness, wit’ (Charlton T. Lewis, Charles Short, A Latin Dictionary).

The Latin phrase is found in English literature in the 1600s and 1700s, and salis appears to precisely mean ‘good sense, intelligence’. For instance in this 1670 Truth Rescued from Imposture by a self-described “profest Enemy to Oppression” (my boldface in all quotes):

This is a wretched and uncharitable construction of the Recorder’s words. These words do no wayes justifie the Papists, if these Libellers had but the least grain of Charity, they would have construed the words, cum grano salis, as the Rule of Charity directs all words to be construed.

Also in Nathan Bailey, Dictionarium Britannicum Or a More Compleat Universal Etymological English Dictionary, London, 1736:

Ill will never spoke well

The use we make of this proverb is, that when we know any person bears a grudge to another, we are not always to believe whatever he says to his disadvantage, but to receive it, according to the Latin proverb: Cum Grano Salis.

It looks as though 1600s English salt may have borrowed this meaning from Latin outside the phrase with a grain of salt too. The following is from the Fables of Æsop, London, 1694. A fox is examining a finely carved head:

[…] Well, (says he) What Pity ‘tis, that so Exquisite an Outside of a head should not have one Grain of Sense in’t.

The Moral

‘Tis not the Barber or the Taylor that makes the Man; and ‘tis No New Thing to see a Fine Wrought Head without so much as One Grain of Salt in’t.

And this is from John Bramball, Castigations of Mr. Hobbes, 1658:

And from thence according to his wild roving imaginations, he draweth consequences from the staff to the corner, that have not the least grain of salt, or weight in them.

A popular hypothesis (Wikipedia and The Phrase Finder) has the phrase originating in Pliny the Elder’s Naturalis Historia. In chapter 77, on the medicinal use of walnuts, he gives a recipe for poison antidote that includes a grain of salt (“addito salis grano”). The recipe would have been found by Roman general Pompey in the tent of the defeated Pontian king Mithridates. In another account Pompey would just take a grain of salt to help him swallow foul-tasting medicines (Wikipedia). You may want to take these with a grain of salt.

Solution 3:

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!

Solution 4:

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.