It means not a proper doctor. Someone unqualified but acting as if qualified.
An example of a quack doctor is the faith healer.
a quack.
The doctor is Dr Quack!
quack (as in quack doctor) is 'un charlatan' in French. To quack (for a duck) is translated by the verbs 'cancaner' or 'craquetter'.
quack
quack
A bad doctor can be called a quack.
"Quack" can be used as a noun to refer to the sound a duck makes or as a term to describe someone pretending to be a medical professional. For example, "The duck let out a loud quack" versus "It's dangerous to trust your health to a quack doctor."
Did it quack like a duck? That doctor is a quack. One "Quack!" from the duck was all it took to startle the curious puppy.
quack
A fake doctor is sometimes called a quack.
It is quack.