Whay does it mean? How would you say it another way?
You haven’t got a grain of sense. or You don't have a grain of sense.

Does it mean;

You are not smart.

Question

Whay does it mean? How would you say it another way?
You haven’t got a grain of sense.  or  You don't have a grain of sense.

Does it mean;

You are not smart.

Answer

Both sentences are fine. It doesn't necessarily mean you aren't smart, but that you lack common sense. You might have a lot of knowledge or education, but you are foolish in other ways. You could also say "You haven't got an ounce of sense."