The expression "quacks" derives from "quacksalver" or quicksilver (mercury) which was being prescribed for various ailments by some very unscrupulous "healers" way back when. Mercury is highly poisonous. As the nefarious activities of these charlatans were gradually brought to light, they became referred to as "quacks" because of the quicksilver they were peddling to their unsuspecting victims.

NO connection with ducks whatsoever, in fact.

- LH