quack: English has two words quack. The one denoting the call of a duck [17] originated of course as an imitation of the sound itself. Quack ‘person claiming to be a doctor’ [17] is short for an earlier quacksalver, which etymologically denoted ‘someone who prattles on or boasts about the efficacy of his remedies’. It was borrowed from early modern Dutch quacksalver, a compound formed from the now obsolete quacken ‘chatter, prattle’ and salf, the Dutch relative of English salve. [quack etymology, quack origin, 英语词源]