The sound that ducks make; or the action of making that distinctive loud, harsh sound; can also refer to a fake doctor.
From the imitative word 'quack,' which has been used since the 17th century to represent the sound ducks make. The meaning extended to 'quack doctor' from Dutch 'quacksalver,' someone who talked loudly about fake medical cures.
Ducks are particularly vocal animals, and 'quack' is one of the few animal sounds that actually appears in the dictionary under multiple meanings—animal noise, and fraudulent doctor—showing how the same word can branch into completely different semantic territories based on cultural and linguistic history.
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