Doctors or healers who are fake or unqualified, pretending to have medical knowledge they don't actually have; also the sounds ducks make.
From Dutch 'kwakzalver' (quack salver)—'kwak' (loud noise) plus 'zalf' (salve/ointment). Originally described charlatans who shouted loudly about their fake medicines. The duck sound comes from imitation of the actual quacking noise ducks produce.
The word 'quack' perfectly captures how con artists worked—they'd shout loudly about their miraculous cures to distract from the fact that their 'medicine' was basically mud. Modern quacks moved to the internet, but the strategy is identical: make noise, sell BS, disappear!
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