1. The sound of a duck: This is the most common meaning. It refers to the distinctive, loud vocalization that ducks make.
2. A person who pretends to have medical knowledge or skill: This meaning is derogatory and refers to someone who practices medicine without proper qualifications. It implies deception and lack of expertise.
3. To speak or act in a way that is insincere or dishonest: This meaning is similar to the second one but can also refer to someone who is simply trying to impress or mislead others.
4. A type of medicine: "Quack medicine" refers to remedies and treatments that are not scientifically proven or supported. This meaning is often used in the same context as the second and third meanings.
The meaning of "quack" will be clear based on the context in which it's used.