It means the same as 'not all there' = mentally incompetent; of low intelligence; absentminded. Out to lunch A brick short/shy of a load Not the sharpest knife in the drawer

Above answers are correct. You can substitute it with other things as well, such as 'an apple short of a bushel'.

A sandwich short of a picnic: stupid, not intelligent, not smart It's an idiom; a phrase or sentence that has a hidden meaning. Difficult to learn but very useful. Many idioms are used often by native speakers. HtH Rob

In this case, "short" has the meaning of "not enough". If you are short of money, it means you don't have enough money. Literally, "one sandwich short of a picnic" means you are lacking one "sandwich" to have a complete picnic. In this idiom, it means you are stupid - you have some mental capacity, but definitely not enough to be logical or sane. In addition to the other idioms above, there is "a beer short of a six-pack", with the same meaning.