N-COUNT 庸医；江湖郎中；冒牌医生 If you call someone a quack or a quack doctor, you mean that they claim to be skilled in medicine but are not.

ADJ 江湖医术;骗人的医术 Quack remedies or quack cures are medical treatments that you think are unlikely to work because they are not scientific.

VERB (鸭子)发出嘎嘎声 When a duck quacks, it makes the noise that ducks typically make.