as nutty as a fruitcake

Images acquired from Wikimedia Commons and used with permission. Left image attribution: Stu Spivak. Right image attribution: David de la Luz.

Explanation:

First, in American English, people can call someone a "nut", a "nutcase", or a "fruitcake" if he or she is crazy. You can also use the adjective "nutty" to say the same thing. Now, a fruitcake is literally a kind of cake (see above image) that Americans give to each other during the winter holidays. It looks like a brick, and it tastes twice as bad. It has, embedded in it, different kinds of nuts as well as multicolored pieces of an unknown substance which looks like fruit.

So, the expression "as nutty as a fruitcake" refers to someone who is very crazy. When I say very crazy, I'm not talking about some guy who gets drunk, then walks around naked. I'm talking about the kind of guy who runs around in the street, screaming and biting people, until the police come and throw a net over him! Check out the werewolf (wolfman) picture above to see what this kind of guy looks like.

When someone is so crazy that other people run away as soon as they see this person, then we say that he or she is as nutty as a fruitcake.

Example:

Joe: Hey Diane. What's wrong? You look like you've just seen a monster!

Diane: I have! It's Gary! He just changed into a werewolf, and now he's going crazy! He's throwing stuff, and busting up the office!

Joe: Oh no! Gary has a special medical condition. He can't work at night during a full moon. Otherwise he changes into a werewolf!

Diane: You'd better call the police, and fast!

Joe: You're right we need their help!

Joe picks up the phone and calls the police.

Officer Copman: Police department. Copman speaking. What can I do for you?

Joe: Officer, we've got this guy over here who just changed into a werewolf, and he's acting crazy and destroying the whole office! He's as nutty as a fruitcake!

Officer Copman: Is he still in your immediate area? Can you see him?

Joe: Yeah, I can see him. He's breaking things, and now he just threw somebody out the window!

Officer Copman: Ok. We'll send some officers over immediately to take him away.

Joe: Good. I'm sure he'll get the medical treatment he needs after you take him to the hospital.

Officer Copman: The hospital? We can't take him there; he's a crazy werewolf. We're taking that guy straight to the zoo!

Would you like a translation of the conversation above into your native language? Then you should check out Babylon's translation software (aff) which can translate between English and many other languages.