没事儿。

You can use it as a standalone answer. If someone bumps into you, say méishìr. If someone thanks you for a small favor, say méishìr. It is incredibly flexible. You can also add it to the end of a sentence to reassure someone. It acts like a verbal shrug that says, 'Don't sweat the small stuff.'

Use it when someone apologizes for being two minutes late. Use it at a restaurant if the waiter forgets your napkin but you already found one. It is perfect for texting friends when they ask if you are mad. In a professional setting, use it to show you are easy-going about minor changes. It makes you sound relaxed and fluent.

Do not use it for major life disasters. If someone crashes your car, méishìr sounds sarcastic or weirdly robotic. Avoid using it with very high-ranking bosses if they are offering a serious apology. In those cases, a more formal 'don't mention it' is better. Also, if someone is actually hurt, ask if they are okay first before telling them 'it's nothing'!

The 'r' at the end is the famous 'Erhua' sound of Northern China. It makes the phrase sound warmer and more colloquial. In the south, people often just say méishì. The phrase reflects a cultural value of 'keeping the peace' and not making a big deal out of small inconveniences. It is the linguistic version of a friendly pat on the back.

You will hear méiguānxi which is the textbook version of 'it doesn't matter.' There is also méiwèntí which specifically means 'no problem' as in 'I can do that task.' If you want to sound very cool and Beijing-local, really lean into that final 'r' sound. It should roll off your tongue like a little purr.