When combined, 沆 (night fog) and 瀣 (night dew) create an image of the damp, heavy air of the night. 一气 (one breath) suggests they are blending and moving as one. The literal imagery is poetic: “the night fog and dew blend into one single breath.” This originally described people with similar tastes and temperaments, but has since evolved to describe people with similarly bad tastes and temperaments.
The story behind 沆瀣一气 is a fascinating example of how a word's meaning can completely flip over time. Originally, during the Song Dynasty, a scholar named Cui Ya was criticized for being recommended by another official with the same surname. The critic sarcastically called them “沆瀣一气,” implying they were like-minded individuals. However, the person who said it actually intended it as a compliment, comparing their noble characters to the pure, blended mists of the night. Over centuries, this meaning was lost. The association of “night mist” and “blending together” took on a more sinister, secretive connotation. Today, the idiom is almost exclusively derogatory. A common Western equivalent is “birds of a feather flock together,” but there's a key difference. The English phrase can be neutral or even positive (e.g., “All the artists hang out at that cafe; birds of a feather flock together.”). 沆瀣一气, in modern usage, is always negative. It doesn't just mean people are similar; it means they are similar in a bad way and are actively conspiring. It carries a strong sense of moral judgment that the English phrase often lacks.
This is a formal and literary idiom. You won't hear it used lightly in casual conversation unless someone is being dramatic or making a formal accusation.
The connotation is always negative and accusatory. Using this term is a serious charge, implying that the subjects are morally corrupt and actively working together to cause harm.