Green’s Dictionary of Slang

headcase n.

[SE head + case n.1 (2d)]

1. an eccentric, bizarre person.

[UK]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.
[US]C. Hiaasen Skin Tight 64: This broad is creepy, he thought, a real head case.
[US]D. Lehaine A Drink Before the War 25: Ann must have been dealing with a real headcase.
[UK]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.

[UK]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.
[Ire]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.
[UK]T. Wilkinson Down and Out 160: He’s a nutter, a head case.
[UK]Indep. on Sun. 10 Oct. 11: If they took all the druggies and headcases out of Brixton, the place would be empty.
[Ire]P. McCabe Emerald Germs of Ireland 305: A complete and utter bucking headcase!
[UK]G. Malkani Londonstani (2007) 167: Lots of hard cases were also head cases.

3. a violent person, a psychotic.

[UK]W. Hall Long and the Short and the Tall Act I: You’re just a head case.
[UK]B. McGhee Cut and Run (1963) 88: Your man’s a bit o’ a head-case, eh?
[UK]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.
[Ire]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.
[UK]N. Barlay Curvy Lovebox 116: Notice two of his Fan Club headcases laggin’ at the door.
[UK]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.
[US]C. Goffard Snitch Jacket 99: I began to grasp the appeal of a headcase like Charlie Manson.

4. a state of psychosis.

[US]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.

[UK]C.P. Taylor Happy Days Are Here Again (1968) 153: Everybody is sick of you, Liphitz! [...] Your intellectual headcases are sick of you.