cash cow：摇钱树
a cash cow is a business that is very profitable.利润丰厚的项目。
the new movie theater that they just opened by campus is a real cash cow.
学校旁边新开的那家电影院真是个摇钱树。
play the lute to a cow: 对牛弹琴
也写作to cast pearls before swine/ preach to deaf ears
指那些说话或写东西不考虑到听众的人。 一般来说，是说话人或作者高估了听众或读者，所以，这则成语嘲讽的主要是听众或观众，而不是说话者。