foul one's (own) nest

The idiom "foul one's own nest" has its origins in the behavior of birds who will dirty their own nests with excrement or waste. The phrase is now used in a more figurative sense to describe someone who is behaving in a way that is damaging or harmful to their own reputation or interests. The usage of the idiom is not limited to any particular context and can be applied to a variety of situations where someone is harming themselves or others by their behavior.