From Longman Dictionary of Contemporary Englishquackquack1 /kwæk/ verb [intransitive] CHBAto make the sound a duck makes→ See Verb table
Examples from the Corpus
quack• She began a frantic quacking at the moment that Cyril got his two hands around her.• Young kids used to quack at us as we went past.• The water is full of thrashing, quacking excitement.• He gobbled, be quacked, grunted, swallowed syllables.• About a dozen mallard ducks were sleeping on the sand or quacking in a subdued manner under the dock.• Clearly, enough investors think that Donald Duck will quack on into the twenty-second century!• One wing's shredded, one's flapping, it's quacking up a storm.quackquack2 noun [countable] 1 Cthe sound a duck makes2 MAsomeone who pretends to be a doctor – used to show disapproval → quackery quacks selling weight-loss drugs3 British English informalMN a doctor You’d better go and see the quack with that burn.Examples from the Corpus
quack• You may be considered an elitist or a quack.• But when they open their mouths the same old hacking quacks come out, the same old self-serving screeds.• There is always some magic remedy that will cure it, or some whizz-kid quack with a patent method.• If she mentioned paranoia, Buzz would insist that Elinor was the victim of a lot of quacks.• Larry paid some quack over a thousand dollars to cure his insomnia.• That quack doesn't know anything about treating heart disease.quackquack3 adjective [only before noun] relating to the activities or medicines of someone who pretends to be a doctor → quackery a quack remedyExamples from the Corpus
quack• Many of those quack doctors were busy selling their own, often more dangerous diet cures.• a quack remedy for coldsOrigin quack1 (1600-1700) From the sound quack2 1. (1800-1900) → QUACK12. (1600-1700) quacksalver “quack” ((16-19 centuries)), from early Dutch, probably from quacken “to quack, talk” + salf “cure” 