Welcome to English in a Minute, where we teach you all about idioms in American English.
Let's pretend you are in a major U.S. city. You are listening to an American English conversation, and you hear this:
A: Hey,Bob. Ever since you opened your coffee shop, you have been very busy. Is the business going well and making money?
B: Yeah! I can't believe it. It's going really well. In fact, my coffee shop has become a real cash cow for me.
A: How does a cow produce cash? Everyone knows cows make milk. But in American English, a "cash cow" is a product or service that makes a lot of money for a business. This common phrase is used in casual and business conversations. And when you use it, everyone will understand what you are saying.
And that's English in a Minute.