one taco short of a combination plate

One taco short of a combination plate is another way of saying someone is not very smart or not all together. By saying someone is a taco short of a combination plate; you are saying this combination plate is not complete because it is missing an important element. Figuratively, this means that a person does not have everything together, which is another way of saying they are missing a part of their brain that leads to a lot of intelligence.