Green’s Dictionary of Slang

fruitcake n.1

[they are nuts adj. (2)]

an eccentric, a peculiar person; esp. in phr. nutty as a fruitcake; thus fruitcake factory n., a psychiatric institution.

[US]E. O’Neill The Movie Man in Ten ‘Lost’ Plays (1995) 187: We sure are as nutty as a fruitcake or we wouldn’t be here.
[US]W. Winchell On Broadway 21 Oct. [synd. col.] ‘You’re nuttier than a fruitcake’ has only been in nine movies this year.
[US](con. 1942) J.H. Burns Gallery (1948) 182: It seemed that Lieutenant Almeranti had gone nuttier than a fruitcake.
[US]B. Schulberg On the Waterfront (1964) 192: Boy, what a fruitcake you are!
[Aus]P. Pinney Restless Men 73: ‘Bloody fruitcake,’ Charlie rasped.
[Can]R. Caron Go-Boy! 79: Get lost, you old fruit-cake!
[Aus]R. Aven-Bray Ridgey-Didge Oz Jack Lang 28: Fruit Cake Factory Lunatic asylum.
[UK]Viz June/July 41: The Queen is a fruitcake claims former palace man.
[Aus]T. Winton Human Torpedo 106: He’s a pacifist, they told each other. A fruitcake, but you gotta respect him for it.
[US]N. McCall Makes Me Wanna Holler (1995) 278: That critical shortage is [...] driving them crazy, as nutty as fruitcakes.
[UK]B. Hare Urban Grimshaw 182: They’re all fucking fruitcakes.
[UK]Observer Rev. 26 Jan. 7/1: There are clearly one or two fruitcakes still around.
[US]J. Porter ‘Good Luck in Puerto del Oro’ in ThugLit July [ebook] ‘A bunch of fruitcakes in hoods’.

In phrases

fruitcake around (v.)

(US black) to misbehave, to mess around.

[US] ‘Idioms of the Present-Day American Negro’ in AS XIII:4 Dec. 314/2: FRUIT CAKIN’ AROUND. Doing something slightly naughty.