Green’s Dictionary of Slang

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

1914		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.
1933		W. Winchell On Broadway 21 Oct. [synd. col.] ‘You’re nuttier than a fruitcake’ has only been in nine movies this year.
1947		(con. 1942) J.H. Burns Gallery (1948) 182: It seemed that Lieutenant Almeranti had gone nuttier than a fruitcake.
1955		B. Schulberg On the Waterfront (1964) 192: Boy, what a fruitcake you are!
1966		P. Pinney Restless Men 73: ‘Bloody fruitcake,’ Charlie rasped.
1978		R. Caron Go-Boy! 79: Get lost, you old fruit-cake!
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 28: Fruit Cake Factory Lunatic asylum.
1989		Viz June/July 41: The Queen is a fruitcake claims former palace man.
1990		T. Winton Human Torpedo 106: He’s a pacifist, they told each other. A fruitcake, but you gotta respect him for it.
1994		N. McCall Makes Me Wanna Holler (1995) 278: That critical shortage is [...] driving them crazy, as nutty as fruitcakes.
2005		B. Hare Urban Grimshaw 182: They’re all fucking fruitcakes.
2014		Observer Rev. 26 Jan. 7/1: There are clearly one or two fruitcakes still around.
2014		J. Porter ‘Good Luck in Puerto del Oro’ in ThugLit July [ebook] ‘A bunch of fruitcakes in hoods’.
2024		J. Meades Empty Wigs (t/s) 670: He wrote ‘Fruitcake wants mitre’ on the dossier.