A pejorative phrase meaning not very intelligent or of questionable mental capacity. This expression can appear in many different forms and variations (e.g., several bricks short of a load," "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 one sandwich shy of a picnic. The new manager is nice enough, but he's one sandwich shy of a picnic, if you ask me.