狗嘴里长不出象牙 meaning in Chinese

no ivory will come of a dog's mouth.; a cracked bell can never sound well.; a filthy mouth can't utter decent language.; no ivory issues from the mouth of a dog; one cannot expect to find elephant tusks in the mouth of a dog -- a mean fellow never speaks nice things.; one doesn't expect ivory from a dog's mouth.; you can't make a silk purse out of a sow's ear.; you don't find ivory in a dog's mouth