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.