The characters literally combine to mean “stinky smells are mutually compatible.” This powerful, almost visceral imagery creates the idiom's meaning: people whose negative qualities or “moral stink” are the same are naturally drawn to one another.
`臭味相投` reflects a strong cultural emphasis on the moral quality of relationships in Chinese society. A friendship is often judged by the character and virtues of the people involved. This idiom serves as a tool for social criticism, sharply condemning alliances based on negative or dishonorable common ground. It stands in stark contrast to the Western concept of “birds of a feather flock together.” While the English phrase is a neutral observation that similar people tend to congregate, `臭味相投` is inherently judgmental. You could say “They both love classical music, so they're birds of a feather,” but you could never use `臭味相投` for such a positive connection. The closest English equivalents are phrases like “thick as thieves” or “partners in crime,” both of which imply a bond based on something illicit or conspiratorial. However, `臭味相投` can be used for less criminal, but still undesirable, traits like laziness, gossip, or vulgar taste.
This idiom is potent and is used to express strong disapproval.