Green’s Dictionary of Slang

1. an eccentric, bizarre person.

1965		L. Dunne Goodbye to The Hill (1966) 81: I was a right head case, I know, but I cared about how much carboniferous limestone was in one country and how much rain fell annually in another.
1989		C. Hiaasen Skin Tight 64: This broad is creepy, he thought, a real head case.
1994		D. Lehane A Drink Before the War 11: Ann must have been dealing with a real headcase.
2009		J. Joso Soothing Music for Stray Cats 153: [I] found myself stuck [...] with my very own head-case.

2. someone undergoing, or in need of, psychiatric treatment.

1965		L. Dunne Goodbye to The Hill (1966) 65: The next time we went for a drink he ordered whiskey on the rocks and I thought he was a head case until I saw the lumps of ice in his glass.
1976		P. O’Farrell Book of Irish Soldiers’ Jokes 48: [Soldiers’ Alphabet] P for relief / Q for lunch / R for tea / S for you! You’re a head case.
1981		T. Wilkinson Down and Out 160: He’s a nutter, a head case.
1999		Indep. on Sun. 10 Oct. 11: If they took all the druggies and headcases out of Brixton, the place would be empty.
2001		P. McCabe Emerald Germs of Ireland 305: A complete and utter bucking headcase!
2006		G. Malkani Londonstani (2007) 167: Lots of hard cases were also head cases.
2012		T. Piccirilli Last Kind Words 111: I was a head case. Jealousy ripped through me. That angry child’s cry of I want, I want.
2020		D. Winslow ‘The San Diego Zoo’ in Broken 126: [A] wacko, a sick headcase [...] who wanted to see what would happen if you gave a handgun to a chimp.
2021		P. Papathanasiou Stoning 257: ‘Omari, eh? What a headcase’.

3. a violent person, a psychotic, also attrib.

1959		W. Hall Long and the Short and the Tall Act I: You’re just a head case.
1962		B. McGhee Cut and Run (1963) 88: Your man’s a bit o’ a head-case, eh?
1982		P. Redmond Tucker and Co 9: They’d already had one brush with Gripper Stebson, now the unchallenged school headcase, and Jonah didn’t fancy the idea of another.
1996		P. Howard The Joy (2015) [ebook] He’s a bleedin headcase, with a love of knives and a lust for using them to cut people up, including himself.
1997		N. Barlay Curvy Lovebox 116: Notice two of his Fan Club headcases laggin’ at the door.
2000		Guardian Guide 12–18 Feb. 52: He’d only get done over [...] by some young thug or local headcase wanting to prove how hard he was.
2007		C. Goffard Snitch Jacket 99: I began to grasp the appeal of a headcase like Charlie Manson.
2013		T. Robinson Hard Bounce [ebook] I tried to find the crack in this personality. One of them — the happy kid or the head-case young woman — had to be a façade.
2017		L. McInerney Blood Miracles 133: ‘Who am I to judge?’ ‘You couldn’t judge him as being a fucking headcase?’.
2018		I. Welsh Dead Man’s Trousers 105: But he’ll get his [...] Cunt’s a headcase.

4. a state of psychosis.

1981

M. Baker Nam (1982) 197: You did your duty, you didn’t run off to Canada. You didn’t fake some head case to go 4-F.

5. a clever person, or one who believes themselves to be so.

1967		C.P. Taylor Happy Days Are Here Again (1968) 153: Everybody is sick of you, Liphitz! [...] Your intellectual headcases are sick of you.
2017		S.M. Jones August Snow [ebook] Then there were egocentric head cases like Atchison who enjoyed a game of chess with their potential executioner.