Green’s Dictionary of Slang

loon n.1

also lont, loun
[SE lunatic; ult. Lat. luna, moon; such people are supposedly ‘moonstruck’; however further ety. suggests a link to the aquatic bird the loon, its ‘craziness’ referring to its actions in escaping from danger and its ‘mad’ cries. Note mid-15C+ SE loon, a worthless person, a rogue, an idler]

an insane, unstable individual.

[UK]Glasgow Herald 27 Mar. 4/1: His run-awa’ daughter that had decampit wi’ some neerdowell loon of a half-pay captain.
[US]N. Ames Mariner’s Sketches 195: The Peruvian war brig [...] which these hen-hearted loons had taken for a pirate.
[US]‘Major Jones’ Sketches of Travel 102: They was every one either drunk or crazy as loons.
[UK]E.V. Kenealy Goethe: a New Pantomime 192: Lobcock, loon, slabberdegullion!
[US]Melville Moby Dick (1907) 447: Peace, thou crazy loon.
[US]‘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.
[UK]Stirling Obs. 12 Sept. 5/6: A loon [...] had cut up his auld leather breeks and biled them.
[UK]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’.
[UK]E. Pugh Spoilers 119: He had known a half-witted loon named Walter.
[UK]Magnet 27 Aug. 27: You’re an ill-bred, cantankerous loon.
[US]J. O’Connor Broadway Racketeers 130: ‘Petie the Priest’ [....] was a trifle balmy. He was looked upon as a religious loon.
[UK]Wodehouse Right Ho, Jeeves 212: Go away, you crazy loon!
[Aus]N. Lindsay Age Of Consent 214: Not your fault; can’t argue with a bloody old loon like that.
[US]R. Chandler Lady in the Lake (1952) 46: He was bending far out over the railing, staring down like a loon.
H. Williams ‘Howlin’ at the Moon’ [lyrics] Now I can’t tell day from night, I’m crazy as a loon.
[US]C. Himes Crazy Kill 138: ‘Crazy as a loon,’ Grave Digger said.
[US]J. Charyn Once upon a Droshky 62: She looks at me like I’m a regular loon. Who can blame her?
[UK]A. Sillitoe Start in Life (1979) 258: What would I do in first class, you great loon?
[US]J. Ellroy Brown’s Requiem 179: Walter was whooping and yelling like some new alcoholic species of loon.
[US]B. Hamper Rivethead (1992) 161: Were these the same deplorable loons [...] that had shamelessly spermatized the Howie Makem saga?
[UK]A. Warner Sopranos 27: Fucksake Kylah, you loon!
[US]T. Dorsey Hurricane Punch 79: Florida street loons didn’t faze him anymore.
[UK]J.J. Connolly Viva La Madness 200: An upper-class loon called Dougie Nightingale.
[Aus] 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.
[US]New Yorker 11 Aug. [Internet] He wasn’t dope-sick or a flat-out loon.