"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:
- An idiom that means that one should consider the subject discussed with appropriate skepticism
- One should make a RealityCheck with respect to the subject discussed
- Use your own discretion with respect to the subject discussed
The similar-sounding but distinct "pinch of salt", generally used in a cooking context, means tweak until optimal.
This is the smallest possible unit of untruth.
(Note: Unsupported. Possibly a TerryPratchett definition.)
See: GrainOfTruth, AmericanCulturalAssumption, BritishCulturalAssumption