There are good quacks and bad quacks. A good quack is the sound a duck makes. A bad quack is someone pretending to be a doctor. (You'd be better off visiting the duck with your ailment.)
If you ever get a chance to interview a celebrity duck, prepare to write the word quack in your notebook many, many times, because that’s all that ducks can say. The way you spell animal sounds changes depending what language you speak. In English a duck quacks, but in Danish a duck "raps," while Indonesian ducks "wek," and Romanian ducks say "mac." And that "MD" who isn’t really a doctor, but is good at pretending to be one? He's a quack. If you ever meet a quack that "quacks," please run.
Primary Meanings of quack
the harsh sound of a duck
utter quacking noises
an untrained person who pretends to be a physician and who dispenses medical advice
act as a medical quack or a charlatan