Meaning:

“已读” means the act of having read a message, and “不回” means giving no response. Together the phrase means that no reply has been given after reading a message.

Left on read, or read but no response, is typically referring to text messages, as well as messages on social media platforms. Some of those platforms enable users to be notified when a message has been viewed.

The expression dates back to at least 2016. A number of popular publications online discussed left on read in the context of dating and romantic relationships, where the frustrations of being left on read can be particularly keen.

Example:

A: 你收到昨晚我发给你的信息了吗？

Nǐ shōudào zuówǎn wǒ fā gěinǐ de xìnxī le ma?

Did you receive the message I sent you last night?

B: 呃...... 收到了，但我忘记回复你了。有什么重要的事情吗？

E… shōudàole, dàn wǒ wàngjì huífù nǐ le. Yǒu shénme zhòngyàode shìqing ma?

Ooh … yes, I did. I read it and then I totally forgot to answer you. Was it something important?

A: 我真不敢相信，你居然已读不回！

Wǒ zhēn bùgǎn xiāngxìn, nǐ jūrán yǐdúbùhuí!

I can’t believe it! I got left on read!