The characters combine literally and powerfully: “not speak/use reason.” The phrase describes someone who has stepped outside the bounds of a logical exchange.
In Chinese culture, there's a strong social emphasis on 道理 (dàolǐ), or “reason/principle.” A proper discussion or argument should follow a clear line of logic. To be “reasonable” (讲道理, jiǎng dàolǐ) is a virtue, showing that you are a rational and considerate person. Consequently, accusing someone of being 不讲理 (bù jiǎng lǐ) is a significant criticism. It suggests they are not just wrong, but are actively violating the social contract of rational interaction. It implies they are causing disharmony by being stubborn and refusing to see things from a principled standpoint. Compared to the Western concept of being “unreasonable,” 不讲理 carries a heavier weight of social transgression. In the West, being “unreasonable” can sometimes imply being quirky or driven by passion. In China, 不讲理 is almost always a negative judgment of someone's character in a specific situation, framing them as difficult, selfish, or even childish.
不讲理 is an extremely common, informal term used to express frustration in everyday life.
The connotation is consistently negative and informal. While you might complain to a colleague that a client is 不讲理, you would use more formal language in a written report.