Definition of "loon"

loon¹

/ˈluːn/

noun
plural loons
1. (slang) A crazy or deranged person; a lunatic.
  Quotations
  An electric fire came next, followed by an umbrella and then a colander. "This bowl will carry no water," he muttered. "Some loon hath pierced it with holes."
  1971, Richard Carpenter, Catweazle and the Magic Zodiac, Harmondsworth: Puffin Books, page 67
  What's it like to be a loon? / I liken it to a balloon
  1971, Marc Bolan (lyrics and music), “Cosmic Dancer”, in Electric Warrior, performed by T. Rex
2. (obsolete) An idler, a lout.
3. (chiefly Scotland, Ulster) A boy, a lad.
4. (chiefly Scotland) A harlot; mistress.
5. (chiefly Scotland) A simpleton.
6. (Ireland, historical) An English soldier of an expeditionary army in Ireland.
7. (traffic engineering) A round area of pavement that protrudes from one side of a road to accommodate turning vehicles with a wide turning circle.

loon²

/ˈluːn/

noun
plural loons
1. (US, Canada) Any of various birds, of the order Gaviiformes, of North America and Europe that dive for fish and have a short tail, webbed feet and a yodeling cry.
  Quotations
  The Loone is an ill ſhap'd thing like a Cormorant; but that he can neyther goe nor flye; he maketh a noiſe ſometimes like a Sovvgelders horne.
  1634, William Wood, “Of the Birds and Fowles both of Land and Water”, in New Englands Prospect. A True, Lively, and Experimentall Description of that Part of America, Commonly Called New England; […], London: […] Tho[mas] Cotes, for Iohn Bellamie, […], 1st part, page 31
  [O]h, yes! the loon does shriek dreadfully - particularly when there's fine rain […]
  1886, Peter Christen Asbjørnsen, translated by H. L. Brækstad, Folk and Fairy Tales, page 286