There are many ways to say that a person isn’t very bright in English. This page lists many of these, including a number of variants on this snowclone:
A few screws short of a hardware store.
A few cards short of a deck.
A few fries short of a Happy Meal.
A few peas short of a casserole.
A few keys short of a keyboard.
A few sandwiches short of a picnic.
Y is something that contains a large enough number of X, that if “a few” are missing, it’ll slow the party down. If someone is “a few oranges short of a bushel,” they can still go to market, they just won’t be able to compete as well with the other orange sellers. I believe this metaphorical flexibility makes this snowclone an example of [what I am marking as] the strong definition of snowclone1, as it is more than a “playful allusion” to a particular expression. If there is an original referent for this phrase, I have not been able to locate it.
Mark Liberman discussed this “Snowclone of Foolishness” (or variant on “Full Deckisms“) back in July 2005, providing even more examples, and pointing out other variants on the pattern itself. Shy may appear in place of short (“A few straws shy of a bale”), and a number may appear in place of the a few quantifier (“three pickles short of a barrel”).
People seem to have a lot of fun with this expression, so I’d like to share a few more examples:
a few bananas short of a bushel
a few sprinkles short of a sundae
a few hosannas short of a miracle
a few smarties short of a lollybag
a few beers short of a barrel
1I swear, I will go back over the various discussions of what it means to be a snowclone and write them up, eventually.
The Snowclones Database 
Then there’s:
Not the sharpest bulb in the six-pack.
The OED’s wordhunt appeal tried to antedate “one sandwich short of a picnic” to before 1993, and got it back to 1985. Similar phrases seem to be of Australian origin. In Ireland “one X short of a Y” seems to me to be a good deal more common than “a few X short of a Y”.
A few bricks short of a load. Seems like the one I’ve heard most, as well as earliest.
I always thought it was “a few cards short of a full deck” originally.
i forgot the context, but this is one i came up with a while ago
“A few balls short of a Bravia ad”
(in reference to that ad for the Bravia TVs from Sony where lots of colored bouncy balls were dropped down a San Francisco street)
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