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Quack is the sound a duck makes.
A quack can also mean a fake doctor who tries to sell his medicine to people when they do not really make you better which means they are fake. A quack doctor tries to tell people that they can cure everything but they can not, they just con you into going to them, but they do not cure you, they just make you feel a little bit better. They also take some of the jobs from the doctors who have been to university and studied to become a doctor and then Quacks are just taking their patients away from them.
A reference to the word "quack" was made on television series QI for being proven to be funnier than other farm yard animal noises. Due to the way the word is pronounced it naturally forces people to smile enticing others to smile or even laugh.