be one card shy of a (full) deck

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be one card shy of a (full) deck

To be not very intelligent or of questionable mental capacity. This expression can appear in many different forms and variations (e.g., "a few sandwiches short of a picnic," "one brick short of a load.," etc.). He says he's going to start a business selling bees as pets—I think he may be one card shy of a full deck. The new manager is nice enough, but he's one card shy of a deck, if you ask me.
See also: card, deck, of, one, shy