The duck makes the same sound regardless of the country she is in and, with her waddle, she reminds one always of a pompous person, strutting along, and eternally quack quacking to himself.
That is what the Dutch thought of the charlatans and mountebanks parading around through plague-ridden Europe in the sixteenth century, each proclaiming loudly the virtues of his salve or nostrum.
Accordingly the Dutch named them quacksalvers, ducks quacking over their salves. Neighboring countries thought the term so apt that it was borrowed by Germany, Sweden, and England.
The English, however, soon shortened it to quack, and applied it to any pretender of medical learning and skill.