Few fries short of a Happy Meal phrase informal derogatory
A "Happy Meal" is a children's meal sold at McDonald's fast-food restaurants that usually includes French fries.
Used to indicate someone is not intelligence or of questionable mental capacity
He's a few fries short of a Happy Meal, I'm afraid. Possibly, he can't help you anything.
Extremely stupid
Very stupid, or silly
Stupid or crazy
(n): a fool person,
(adj): stupid, foolish, silly
A stupid person or an idiot