|1.||a grain of salt|
Not to be taken literally, or to be taken with caution or extra common sense.
Etymology is from the Latin phrase "cum grano salis" (with a grain of salt), and also the Italian phrase "avere sale in zucca" (to have salt in your pumpkin (head)). In Italy to have salt in your head means to have intelligence or reasoning abilities.
Do not accept promises from your enemy without a grain of salt.
Use a grain of salt when buying cheap electronics from the flea market.