To take with a grain of salt
Pliny The Elder, Natural History
“In the chambers of the great king Mithridaes, Cneius Pompeius discovered in a private notebook, written in the king’s own handwriting, a formula for an antidote to poison: two dry nuts, the same number of figs, and twenty leaves or rue ground together with a grain of salt added: whoever took this on an empty stomach would be harmed by no poison that day.”
Today we see this expression as a cautionary note to take something as doubtful. The phrase is a comment on the effectiveness of Pliny’s recipe as an antidote to poison.