(A) Few X Short of a Y

What does the idiom “(A) Few X Short of a Y” mean?

Definition:

Crazy, mentally impaired.

For example:

“Ned seems to make sense when you talk to him at first, but the more you listen, the more it seems he’s a few cards short of a deck.”

Notes:

Examples: a few cards short of a deck, a few fries short of a Happy Meal.