The literal meaning, “to wear a green hat,” has no connection to its modern idiomatic usage. The phrase's power comes entirely from its deep-seated cultural and historical associations.
The association between green hats and infidelity is deeply rooted in Chinese history, though the exact origin is debated. The most popular theory dates back to the Yuan Dynasty (1271–1368), when regulations supposedly required the male relatives of prostitutes to wear green hats or headscarves to publicly identify them. This created a lasting social stigma, linking green headwear to a man whose wife was promiscuous or unfaithful. This concept is inextricably linked to the cultural value of 面子 (miànzi) - “face” or social dignity. For a man to be “given a green hat” is a catastrophic loss of face. It implies he is weak, undesirable, and unable to control his own household, which are all sources of immense public and private shame in traditional Chinese social structures. Western Comparison: The direct English equivalent is “to be cuckolded.” However, `戴绿帽子` is far more common in everyday Chinese conversation and pop culture than “cuckold” is in English. Furthermore, there is no Western equivalent where a specific color of clothing carries such a powerful and specific negative meaning. While green can be associated with envy in English (“green with envy”), in China, the combination of green + hat specifically signifies infidelity and nothing else.
`戴绿帽子` is an informal and colloquial term. It is used in gossip, jokes (often cruel ones), online memes, and dramatic scenes in movies and TV shows.