Cast pearls before swine对牛弹琴；白费好意

To Cast pearls before swine的意思是“珍珠头在猪猡前面”。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 dbeautiful 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