Green’s Dictionary of Slang

an insane, unstable individual.

1826		Glasgow Herald 27 Mar. 4/1: His run-awa’ daughter that had decampit wi’ some neerdowell loon of a half-pay captain.
1830		N. Ames Mariner’s Sketches 195: The Peruvian war brig [...] which these hen-hearted loons had taken for a pirate.
1848		‘Major Jones’ Sketches of Travel 102: They was every one either drunk or crazy as loons.
1850		E.V. Kenealy Goethe: a New Pantomime 192: Lobcock, loon, slabberdegullion!
1851		Melville Moby Dick (1907) 447: Peace, thou crazy loon.
1862		‘Artemus Ward’ Artemus Ward, His Book 84: That Jack Fawlstarf is likewise a immoral old cuss, take him how ye may, and Hamlick is as crazy as a loon.
1867		Stirling Obs. 12 Sept. 5/6: A loon [...] had cut up his auld leather breeks and biled them.
1882		Nat. Police Gaz. (NY) 4 Feb. 11/3: All sort of crazy loons.
1895		Shields Dly Gaz. 18 Apr. n.p.: ‘Well, well, skipper,’ said the Sheriff good-naturedly, ‘for a’ that the loons would hae been nane the waur o’ their kail’.
1906		E. Pugh Spoilers 119: He had known a half-witted loon named Walter.
1910		Magnet 27 Aug. 27: You’re an ill-bred, cantankerous loon.
1928		J. O’Connor Broadway Racketeers 130: ‘Petie the Priest’ [....] was a trifle balmy. He was looked upon as a religious loon.
1934		Wodehouse Right Ho, Jeeves 212: Go away, you crazy loon!
1938		N. Lindsay Age Of Consent 214: Not your fault; can’t argue with a bloody old loon like that.
1943		R. Chandler Lady in the Lake (1952) 46: He was bending far out over the railing, staring down like a loon.
1951		H. Williams ‘Howlin’ at the Moon’ 🎵 Now I can’t tell day from night, I’m crazy as a loon.
1959		C. Himes Crazy Kill 138: ‘Crazy as a loon,’ Grave Digger said.
1964		J. Charyn Once upon a Droshky 62: She looks at me like I’m a regular loon. Who can blame her?
1970		A. Sillitoe Start in Life (1979) 258: What would I do in first class, you great loon?
1981		J. Ellroy Brown’s Requiem 179: Walter was whooping and yelling like some new alcoholic species of loon.
1991		B. Hamper Rivethead (1992) 161: Were these the same deplorable loons [...] that had shamelessly spermatized the Howie Makem saga?
1998		A. Warner Sopranos 27: Fucksake Kylah, you loon!
2007		T. Dorsey Hurricane Punch 79: Florida street loons didn’t faze him anymore.
2011		J.J. Connolly Viva La Madness 200: An upper-class loon called Dougie Nightingale.
2012		J.J. DeCeglie ‘Death Cannot Be Delegated’ in Crime Factory: Hard Labour [ebook] I had to bust out of there [...] that or run out of the house like a loon with my dick in my hand yelling obscenities.
2015		L. McInerney Glorious Heresies 194: [W]aiting for the post to come around and when it does it’s a letter from a raving loon.
2016		New Yorker 11 Aug. 🌐 He wasn’t dope-sick or a flat-out loon.
2015		L. McInerney Glorious Heresies 18: ‘You can’t leave that loon here’.
2021		L. McInerney Rules of Revelation 40: ‘This is Wild Ireland,’ Izzy said. ‘It’s full of hippies and loons’.