He's one taco short of a combination plate

Meaning

Another colorful way to say someone is not very smart. It suggests that while most of the meal is there, the person is missing one key element.

Playful, mocking

Used in a sentence

• He keeps putting diesel in the unleaded tank, he is one taco short of a combination plate.
• I love him, but he is one taco short of a combination plate.
• Don't get into a debate with him; he is one taco short of a combination plate.