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1.
Meaning to take something lightly, especially with advice. To be skeptical.
Take a novice's advice with a grain of salt.

Anything free should be taken with a grain of salt (there's no such thing as a free lunch)
by philihp September 24, 2004
 
2.
When qualifying advice: For example your mother is always cold and thinks that you will be too. Therefore she always tells you to take a jacket with you. Perhaps she gets cold easily and you do not. If you want to communicate this different subjective preference, use this phrase.
Take your mother's advice to wear a coat "with a grain of salt"
by mthomas1776 October 11, 2007