A duck! 

An aquatic bird that, in medieval times, was used to find out if a person was a witch or not.

This stems from the very logical idea that if a person weighs the same as a duck, then that person is made of wood (because both ducks and wood float in water). And since wood burns (just like witches) then that person must be a witch, because witches are made of wood.

Therefore, the accused person(s) would be placed on a scale next to a duck, and if they balanced, the person(s) would be burned.

So, if she weighs the same as a duck, then she's made of wood. And therefore... A WITCH!

A duck! by Flint October 27, 2003

A duck! 

A form of scientific theory used in the popular movie: Monty Python and The Holy Grail.

We found a witch! May we burn her?
A witch? How do you know she's a witch?
She turned me into a newt!
A newt..?
I got better..
There is a simple way to figure out if she is a witch or not.
Witches burn, and what else burns?
Wood!
Yes, and wood floats. What else floats?
... A duck?
Yes! So if this woman weighs as much as a duck, than she is a witch!

A duck! by WhiteyLights August 4, 2005

A duck! 

What floats on water and weighs the same as a witch.

When asked by Sir Benomir, what also floats, King Authur exclaimed "A duck".

A duck! by Cosmicstargoat January 30, 2004

A duck! 

An insurance salesperson who is continually frustrated by humans who refuse to listen.

aflac, Aflac, AFLAC!

A duck! by Cosmicstargoat June 5, 2004

A duck! 

What looks like a duck, swims like a duck, and quacks like a duck, is a duck (probably).
Where duck is used as a metaphor for something usually totally unrelated to ducks.

"Hey, is that guy a republican?"
"Well, if it looks like a duck, swims like a duck, and quacks like a duck, it's probably a duck."

A duck! by gothfae May 2, 2004

A duck! 

easily tricked

toya, you are such a duck, you believe that i am out of town

A duck! by jakeema November 3, 2003

A duck! 

In the sport of cricket, when a batsman is out without scoring any runs (points).

Also:
Golden Duck - when a batsman is out after only facing one ball

Royal Duck - when a batsman is dismissed by the first ball of the innings

"I didn't score any runs, I got a duck"

A duck! by Corps October 15, 2004