Calque of Latincum grānō salis(Can this(+) etymology be sourced?), literally with a grain of salt, figuratively with a bit of common sense (from Pliny’s Pompey’s discovery of an antidote against poison to be taken with a grain of salt).
I take anything I read on the Internet with a grain of salt.
2008, John Douglas, Johnny Dodd, Inside the Mind of BTK: The True Story Behind the Thirty-Year Hunt for the Notorious Wichita Serial Killer, John Wiley & Sons, →ISBN:
In it, police laid out list of biographical factoids about BTK and urged residents to read it with a grain of salt. Releasing a list of his “claims,” they reasoned, might allow someone, somewhere to make a connection that police couldn't hope to.