Short form of the last part of the famous punchline "But that's not a Duck!" Saying "A Duck!" should always be followed by insane laughter for no perticular reason.
Also can be used as a random exclimation.
Bob and Jack are talking when Paul walks up. Bob suddenly yells "A duck!". Bob and Jack errupt in laughter. Paul is forced to laugh or admit he doesn't understand. It's great fun either way.
November 30, 2004