With reservations or the understanding that some rumor or piece of information may not be completely true or accurate. Possibly a reference to an ancient Roman antidote to poison that included or consisted of a grain of salt. I heard that you can get a free movie ticket if you wear red, but Kevin told me that, so I'm going to take it with a pinch of salt. Read whatever that paper publishes with a pinch of salt—it's really just a trashy tabloid.