Definition of loon

• n. - A sorry fellow; a worthless person; a rogue.
• n. - 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.