Source : Webster's Revised Unabridged Dictionary (1913)

Loon \Loon\, 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.

Loon \Loon\, n. [For older loom, Icel. l?mr; akin to Dan. & Sw.
   lom.] (Zo["o]l.)
   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 ({U.
   septentrionalis}), are the best known species. See {Diver}.

Source : WordNet®

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: {diver}]
     3: a person with confused ideas; incapable of serious thought
        [syn: {addle-head}, {addlehead}, {birdbrain}]