The buldge in a woman's pants. Often seen when a unshaven woman wears a bathing suit.
person one: "That girl looks good in a Bikini"
person two: "yeah, but did you see her walnut?"
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"