A Few Sandwiches Short Of A Picnic

What does the idiom “A Few Sandwiches Short Of A Picnic” mean?

Definition:

Abnormally stupid, not really sane

For example:

“Sometimes I think John’s a few sandwiches short of a picnic. He never seems to understand directions.”

Notes:

There are many variations on this idiom, all having the form “a few ____ short of a ____.”