• Quack is the rather silly noise that a duck makes.
  • Quack is also a term for a deceitful person who pretends to be an expert in some field, especially medicine but doesn't really know very much and makes preposterous claims. A quack may also be someone who knows a great deal and makes preposterous claims to deceive people.

For example, George W. Bush and Donald Trump are quacks! What did you expect? A lot of Conservatives are quacks too! [1]