Idiom
few fries short of a Happy Meal
not very intelligent.I could tell he was a few fries short of a Happy Meal, but unfortunately you can't arrest someone for being stupid.
Usage notes: this idiom appears in many different forms, including a few cards short of a full deck, a few bricks short of a full load, a few clowns short of a circus, and a few Cokes short of a six-pack, all with the same meaning