Someone with strong opinions (especially political) but lacks the eloquence to express them. He makes his stance known only by agreeing with an eloquent speaker with the same opinions.
Teacher: Class, what do you think?
Robert: The problem is with the tax
accountant lobbyists who demand ever more complicated tax codes to give themselves job security and tax payers headaches.
Herbert: Yeah! What he said.
Teacher: Herbert, you are such a dittohead!