Nutty as a fruitcake is a term that refers to someone that is idiotic or crazy.
An example of how to use nutty as a fruitcake in a sentence would be.
John is as nutty as a fruitcake if he thinks he can get that elephant to fit inside of his car.
The adjective nutty meaning "insane" was also first recorded in the year of 1821.
And the similarity to the fruitcake, which literally does contain nuts as well as fruit, was also first recorded in 1935.
When someone says you are a fruitcake it means the person is saying that you are a crazy or eccentric person or a nut.
If you are a very strange or crazy person then someone would refer to you as being a fruitcake or nutty as a fruitcake.
if you do something or are about to do something crazy or idiotic then you would also be nutty as a fruitcake or just be a nut.