as nutty as a fruitcake

As nutty as a fruitcake is a way of saying someone is crazy. Nutty is a slang term often used to describe someone as being crazy, whether they are talking about someone clinically insane or simply silly and a little bit awkward; fruitcakes are known for having a lot of nuts in them (literally) therefore the saying is saying someone is very nutty, i.e. crazy based on how they are, how they speak or how they behave.