Noun. A word tacked onto the end of sentences, regarding the person being spoken to as lower than the speaker in status. A slight insult because it has a condescending tone of superiority. However, the word cannot be used as the topic of a sentence. You're a mofo, does not work. You're an idiot, mofo, works. When I say that the latter works, I mean that it is a correct usage of the word. It does not necessarily work to get you what you want.
Pushy Bothersome Person: Gimme some cheese, mofo.
Apparent Mofo: Um...no. This is my cheese. (The receiver of the word 'mofo' is immediately defensive.)