cash cow
noun [ C ] business specialized uk/ˈkæʃ ˌkaʊ/ us/ˈkæʃ ˌkaʊ/
摇钱树，最赚钱的商品
a product or service that makes a lot of money over a long period of time for the company that sells it, often money that is used to support the company's other activities
The studios discovered that movie rentals were an even bigger cash cow than movie tickets. 电影公司发现，电影租赁是比电影票更大的摇钱树。