The term comes from the literal meaning of dung, which is animal excrement. Since cow dung is often considered smelly and undesirable, the phrase "bull dung" is used to convey the same sense of unpleasantness and unreliability.
Here are some examples of how "bull dung" might be used:
* "That story about the talking dog is bull dung!"
* "Don't tell me bull dung! I know you were the one who ate the last cookie."
* "He's been telling me bull dung all day long about how he's going to become a millionaire."
While it's effective for expressing strong disagreement or disbelief, it's important to remember that "bull dung" is considered vulgar and should be used with caution.