To be not very intelligent or of questionable mental capacity. It can appear in many different forms and variations (for example: a few bricks shy of a load, a few cards shy of a full deck, etc.) He says he's going to start a business selling bees as pets. I think he may be one sandwich short of a picnic.
(humorous) alsobe a couple of sandwiches short of a picnic (humorous)
if someone is one sandwich short of a picnic, they are stupid or crazy After talking to him for about 10 minutes I decided he was definitely one sandwich short of a picnic.