cast pearls before swine
直译：把珍珠投在猪面前
比喻：明珠暗投；对牛弹琴
出自：《马太福音》
Do not give what is holy to the dogs; nor cast your pearls before swine, lest they trample them under their feet, and turn and tear you in pieces.
（Do not give dogs what is sacred; do not throw your pearls to pigs. If you do, they may trample them under their feet, and turn and tear you to pieces.）
不要把圣物给狗，也不要把你们的珍珠丢在猪前，恐怕它践踏了珍珠，转过来咬你们。
我们看个经典作品中的例句：
it's my house—and when I let the upper floor to Cap'en Cuttle, oh I do a thankless thing, and cast pearls before swine!
( Dombey and Son )
这是我的房屋-一当我把楼上租给卡特尔船长的时候，哎呀，我真是做了一件别人毫不领情的事情，简直就等于把珠子扔在猪的面前一样！
