The phrase vividly paints a picture: your precious, pure kindness (`好心`) is mistaken for (`当成`) something foul and worthless (`驴肝肺`).
This idiom reflects a deep-seated cultural value on mutual respect and the proper reciprocation of kindness, known as `人情 (rénqíng)`. When you offer help, there is a social expectation that your good intentions will be recognized, even if the help itself isn't needed. To have those intentions twisted into something negative is a significant social slight. The metaphor is uniquely Chinese and visceral. The choice of “donkey's liver and lungs” isn't random; it's meant to be slightly crude and insulting, representing the lowest-value, most undesirable thing imaginable. This contrasts sharply with the “good heart,” which is seen as sincere and valuable. Comparison to a Western Concept: The closest English idiom is “No good deed goes unpunished.” However, there's a key difference in focus:
This is a very common, informal phrase used to complain or vent frustration.