Although this is an idiom from the United States of America, I do love it.
You use this idiom to describe someone you think is a bit ‘stupid’ or ‘mental’. In England, we use this idiom: To be a sandwich short of a picnic.
My first love at university was definitely ‘a few fries short of a happy meal’. He used to take a plastic bag full of leaves to clubs because he thought that they needed a bit of entertainment. All my mates thought I was ‘a sandwich short of a picnic’ because I used to help him collect the leaves.