the sound that a duck makes （鸭子的）呱呱声，嘎嘎声 - (informal, disapproving)
a person who dishonestly claims to have medical knowledge or skills 江湖郎中；冒牌医生；庸医
Word Originnoun sense 1 mid 16th cent. (as a verb): imitative. noun sense 2 mid 17th cent.: abbreviation of earlier quacksalver, from Dutch, probably from obsolete quacken ‘prattle’ + salf, zalf (see salve).