A brick short of a load, two sandwiches short of a picnic, etc.
Definition 1 of 2

Playful Insult
Often used humorously among friends as a light-hearted jab, not to be taken seriously.

When she forgot the punchline, I teased her for being two sandwiches short of a picnic.
Avoid Offense
Be cautious using it to avoid offending someone, as it can be seen as a rude remark about intelligence.

He didn't realize saying she was a few screws loose in front of everyone would upset her.
Not Literal
It doesn't literally refer to missing items, but suggests someone is behaving unusually.

Aunt May said the neighbor must be a few fries short of a Happy Meal.
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