loon

Etymology 1

From Middle English louen, lowen (“rascal; rogue”), probably of Middle Dutch or Middle Low German origin. Compare Dutch loen (“simpleton”). Or, related to sense 2, due to the bird's loud cry. Folk etymology associates it slang-wise with lunatic, though the latter may have influenced it; see loony.

noun

1. (slang) A crazy or deranged person; a lunatic.
   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.

Etymology 2

Of North Germanic origin, from Old Norse lómr (“loon”), ultimately imitative of the bird's cry, particularly when it's in danger. Distantly related to lament, probably sharing Proto-Indo-European *leh₂- (expressive root).

noun

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.
   [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