Quack/kwæk/ Definition 1 of 4Pro Tip 1/3Negative Connotation Used to express strong disapproval of someone pretending to have medical expertise. Many quacks are exposed on medical TV shows. Pro Tip 2/3Context Sensitive Use in informal contexts when discussing fraudulent or deceptive medical practices. He lost his money to a quack. Pro Tip 3/3Avoid Offense Avoid calling a professional a 'quack' without proof, as it's a serious accusation. Her methods seemed odd, but she wasn't a quack.