A derogatory phrase meaning not very intelligent or of questionable mental capacity. It can appear in many different forms and variations (for example: a few sandwiches shy of a picnic, one card shy of a full deck, etc.). He says he's going to start a business selling bees as pets—I think he may be a few cards short of a full deck. The new manager is nice enough, but he's a few cards short of a deck, if you ask me.