What is the meaning of Quack?

The sound made by a duck.

To make a noise like a duck.

Of a queen bee: to make a high-pitched sound during certain stages of development.

A duck's quack.

A fraudulent healer, especially a bombastic peddler in worthless treatments, a doctor who makes false diagnoses for monetary benefit, or an untrained or poorly trained doctor who uses fraudulent credentials to attract patients

Any similar charlatan or incompetent professional.

Any doctor.

To practice or commit quackery (fraudulent medicine).

To make vain and loud pretensions.

Falsely presented as having medicinal powers.

Source: wiktionary.org