1
general::
Crazy, idiotic, as in Mary's nutty as a fruitcake if she thinks she can get away with that. The adjective nutty meaning "insane" was first recorded in 1821; the similarity to fruitcake, which literally contains nuts as well as fruit, was first recorded in 1935.
American Heritage Idioms
2
general::
Phrase(s): *nutty as a fruitcake [and] nuttier than a fruitcake
crazy. (*Also: as ~.) • Don’t pay any attention to John; he’s nutty as a fruitcake. • Mary’s schemes for making money are nuttier than a fruitcake.
McGrawhill's American Idioms And Phrasal Verbs