Green’s Dictionary of Slang

(US) crazy, eccentric.

1942		S.F. Examiner 12 June 18: If you had lost [...] I guess you would have gone fruitcake.
1959		W.C. Heinz Professional 104: Man, You’re nutty [...] Real fruit-cake.
1972		T. Stoppard Jumpers Act I: As far as I’m concerned it’s got fruit-cake written all over it.
1976		Concentration 17 Sept. [CBS-TV] The audience is going fruitcake, ’cause they know the answer.
1987		C. Hiaasen Double Whammy (1990) 248: All we need is some fruitcake Rambo flashing back to Nam on live TV.
1999		Guardian Guide 22–28 May 52: A prerequisite array of fruitcake American ‘scientists’. [Ibid. ] 12–18 June 25: Fruitcakey Icelanders refusing to skirt risk.
2008		T. Dorsey Atomic Lobster 186: If you’re going to hang out, you can’t act like some fruitcake fan.
2013		N. Knight ‘Not Even a Mouse’ in ThugLit Nov.-Dec. [ebook] ‘Jello. The fuck kinda fruitcake name’s that?’.