The Collaborative International Dictionary of English v.0.48:

Loon \Loon\ (l[=oo]n), n. [Scot. loun, lown, loon; akin to OD.
   loen a stupid man; prob. for an older lown, and akin to E.
   lame.]
   A sorry fellow; a worthless person; a rogue.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Loon \Loon\, n. [For older loom, Icel. l[=o]mr; akin to Dan. &
   Sw. lom.] (Zool.)
   Any one of several aquatic, wed-footed, northern birds of the
   genus Urinator (formerly Colymbus), noted for their
   expertness in diving and swimming under water. The common
   loon, or great northern diver (Urinator imber, or Colymbus
   torquatus), and the red-throated loon or diver (Urinator
   septentrionalis), are the best known species. See Diver.
   [1913 Webster]

WordNet (r) 3.0 (2006):

loon
    n 1: a worthless lazy fellow
    2: large somewhat primitive fish-eating diving bird of the
       northern hemisphere having webbed feet placed far back;
       related to the grebes [syn: loon, diver]
    3: a person with confused ideas; incapable of serious thought
       [syn: addle-head, addlehead, loon, birdbrain]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "loon":
   Boeotian, aliene, bedlamite, block, borderline case, clod,
   crackbrain, crackpot, dement, demoniac, dimwit, dolt, donkey, dope,
   dullard, dumb cluck, dumbbell, dummy, dunce, energumen, fanatic,
   flake, fou, gowk, idiot, jobbernowl, kook, lackwit, lamebrain,
   lightweight, looby, loony, lunatic, madman, maniac, meshuggenah,
   niais, nincompoop, ninny, ninnyhammer, nitwit, noddy, non compos,
   noncompos, nut, phrenetic, put, raving lunatic, screwball, stupid,
   thickwit, weirdo, witling