Not to be taken literally, or to be taken with caution or extra common sense.
Etymology is from the Latin phrase "cum grano salis" (with a grain of salt), and also the Italian phrase "avere sale in zucca" (to have salt in your pumpkin (head)). In Italy to have salt in your head means to have intelligence or reasoning abilities.
Do not accept promises from your enemy without a grain of salt.
Use a grain of salt when buying cheap electronics from the flea market.
The act of mass texting a generic message to members of the opposite sex in hopes that a guaranteed hookup for the night will be established. It often involves a very nonspecific message such as "What are you up to tonight?" or "Want to meet up later?" The key feature is the lack of personalization so that the same message can be sent to as many people as possible. This can also be applied to other contact means such as myspace, facebook or email.
Last night I went booty grazing and had to choose between 3 different hoes.
Old person, usually of grandfather age defined by an ability to nap through nearly any life situation. The term was given to describe the iron grasp such a 'grain grippa' has to their wooden rocking chair during such siesta.
Hit 'em over the head with a beer can or turn up the hit list as loud as you want, that grain grippa ain't gonna wake up for nobody.
When an amish pretends to have money by sliding his hands in the motion in which people "make it rain" for strippers
Amish Guy 1: hey, look man, im making it grain
Civilian Guy 2: wow, maybe you should get a real life and make real moneyfly through the air
(Amish guy 1 runs away to his adobe house with a sack of grain blowing thru the wind)