Not very smart; lacking sufficient intelligence.Generally used when telling someone they are really stupid.
In other words this idiom "indicate in a humorous way that you think someone is very stupid or is behaving very strangely."
1. After talking to him for about 10 minutes I decided he was definitely one sandwich short of a picnic.
2. Poor Bob just isn't too bright.He's one sandwich short of a picnic.
3. She is not stupid but she is just one sandwich short of a picnic