"With a grain of salt" - with reservations; skeptically. e.g.: Take that advice with a grain of salt. Literal translation of the LatinLanguage phrase "cum grano salis", first punt into English in 1647. See for instance http://www.word-detective.com/052598.html .
Definition:
This is the smallest possible unit of untruth.
(Note: Unsupported. Possibly a TerryPratchett definition.)
See: GrainOfTruth, AmericanCulturalAssumption, BritishCulturalAssumption