21.Cast pearls before swine 对牛弹琴；白费好意
To Cast pearls before swine 的意思是“珍珠扔在猪猡前面”。 swine 是个旧词，书面词， 即今为 pigs,不过 swine 单复同行，本句为复数。
这个成语源自《新约。马太福音》第 7 章：“Give not that which is holy unto the dogs,neither cast ye your pearls before swine,lest they trample them under their feet,and turn again and rend you”.由于 to cast pearls efore swine,比喻确切，在后世不断引用中而成为一个国际性成语，常用来表示 to offer sth valuable or beautiful to those who can't appreciate it;to give what is precious to those who are unable to understand its value 等意思，含有轻蔑嘲笑色彩。按其字面意义，这 个成语与汉语成语“明珠按投”相似，但是寓意不同，基本上不对应；按一比喻意义，它相当 于“对牛弹琴”，“向驴说经”“一番好意给狗吃”“狗咬吕洞宾，不识好人心”等。
She read them Shakespeare,but it was casting pearls before swine
I won't waste good advice on John any more because he never listens to it.I won't cast pearls before swine.
...and when I let the upper floor to Cap'en Cuttle,oh i do a thankless thing,and cast pearls before swine。