Fig.: to believe something with reservation; to be doubtful

I'm still so traumatized by the accident that I'm not sure about what really happened, therefore take what I'm saying with a grain of salt

Sono ancora così traumatizzato dall'incidente che non sono sicuro di quanto sia successo, perciò prendete con le molle quello che vi dico

Peter is known to be always exaggerated in everything he does and says, so I would take the confidences he made to you with a grain of salt

Si sa che Pietro è sempre esagerato in tutto quello che fa e che dice, perciò prenderei con le pinze le confidenze che ti ha fatto

Many believe that this expression comes from the Latin sentence "addito salis grano" (plus a grain of salt), a line used by Pliny in his "Naturalis Historia" (XXIII, ch. 77, v 3) to indicate a poison antidote that could work only if taken "with a grain of salt". Others think that the expression may refer to the fact that, in old times, salt was thought to have healing properties and to be an antidote to poison, so to eat or drink something "with a grain of salt" was to practice preventive medicine