Urban Dictionary has a rather large selection of definitions for Duck, some of them include:
A contraction of Dumb F**k
A word in between D*mn and F**k
An undercover cop
An unattractive person
Someone easily influenced
Someone not informed about street life
Someone who talks too much
A person with no sense of style
The use that we've heard as most common is someone that's stupid, taking advantage of the alliteration of Dumb Duck, or Dumb as a Duck.
Just because American English is so common worldwide does not mean that English speakers of different dialects can’t still confuse one another with slang and local terms. American English speakers and British English speakers both have usages that confuse, and amuse one another.
Accents alone can sometimes be enough to form a language barrier, despite the fact that in the U.S., a British accent might be treated as comical depending on the persona it’s attached to. Here is a list of common American slang word and phrases that our English-speaking comrades in Great Britain might have trouble wrapping their heads around.