Few apples short of a fruitcake 

A snowclone to express that someone is either stupid or crazy. The standard format is "a few X short of (a) Y", the idea being that Y is something that, with a few X missing, is not so good, and in the case of a person being compared to such, that something is "missing" in their thought process.
This one in particular has the appeal that fruitcakes involve nuts, thus further solidifying the point that the person is nuts themselves.

Did you see how Bob just flipped out over nothing. Is he having some sort of psychotic break?
Yeah, he seems to be a few apples short of a fruitcake. Might be best to keep our distance.

Few apples short of a fruitcake by Twisted Code March 23, 2017

Few apples short of a fruitcake 

Someone/something below standards.

When someone is a complete asshole, with no real motive.

Same as:
Few fries short of a happy meal
Few yards short of a first down

I asked my boss for my first raise in two years, and he said I deserved it, but I wouldn't get it. He's either a total ass, or a few apples short of a fruitcake.

Few apples short of a fruitcake by hellokoala November 5, 2009