1. The voice characteristic of a water bird with short legs, webbed feet, and a large flat beak: The quack of Trudy's pet duck is the first sound she hears each morning.
2. An individual who pretends to be a medical doctor; someone whose medical practice is suspect or not authorized; a charlatan: Mildred always joked that she was going to see her quack about her arthritis.

That quack almost killed Marvin's brother by prescribing the wrong medication.

To make a noise in order to attract a specific wild bird to a location: The hunter will quack like a duck to lure his prey to the marsh.