It is a term used in dynamic languages that do not have strong typing.
The idea is that you don't need a type in order to invoke an existing method on an object - if a method is defined on it, you can invoke it.
The name comes from the phrase "If it looks like a duck and quacks like a duck, it's a duck".
Wikipedia has much more information.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
