According to the search results, "quack" has multiple meanings, including:

• A practitioner of fraudulent or ignorant medical practices: This is the most common definition of "quack," and it refers to someone who promotes unproven or ineffective medical therapies. The term "quack" is a clipped form of the archaic term "quacksalver," which referred to a hawker of salve in the Middle Ages.

• The sound made by a duck: "Quack" is also the onomatopoeic word used to describe the sound made by a duck.

• An ignorant, misinformed, or dishonest practitioner of any profession: In addition to medicine, "quack" can be used to describe someone who is unqualified and incompetent in any profession.

• A wolf in sheeps clothing: This is an idiomatic use of "quack" to describe someone who pretends to be something they are not.

Overall, the meaning of "quack" depends on the context in which it is used.