The sound made by a duck.
Did you hear that duck make a quack?
To make a noise like a duck.
Do you hear the ducks quack?
A fraudulent healer or incompetent professional; especially, a doctor of medicine who makes false diagnoses or inappropriate treatment; an impostor who claims to have qualifications to practice medicine.
That doctor is nothing but a lousy quack!
A charlatan.
A doctor.
To practice or commit quackery (fraudulent medicine).
To make vain and loud pretensions.
Falsely presented as having medicinal powers.
Don't get your hopes up; that's quack medicine!