The word quack has two completely unrelated meanings:

1. The harsh, flat sound a duck makes. When ducks waddle around a pond, they quack loudly to communicate with each other. Unlike the melodious songs of robins or the cheerful chirps of sparrows, a duck's quack has a distinctive, almost honking quality. Baby ducks make softer, higher-pitched quacks than adult ducks.
2. A fake doctor or someone who dishonestly claims to have medical knowledge they don't actually possess. In the 1800s and early 1900s, quacks traveled from town to town selling bottles of “miracle cure” tonics that were usually just colored water or alcohol. They'd promise their potions could cure many different illnesses, cheating sick people out of their money while offering no real help. Today, a quack might be someone selling fake weight-loss pills online or claiming they can cure serious diseases with unproven treatments. Real doctors spend years studying medicine and must pass difficult exams to earn their licenses. A quack just pretends to have that knowledge, which makes them dangerous to trust with your health.