I was always intrigued by the use of the motherfucker insult in American movies, thinking "well why is there a problem with having intercourse with mothers" but recently I've read a number of pulp fiction crime books from the 30s, in particular ones by black authors. I noted that there are a lot of insults using the term "mother rapers" and I believe that the modern use of the term is actually derived from this - ie. someone who rapes mothers would (at least in my mind) be a derogatory term. It's just been changed, probably within the black community to the seemingly more powerful "mother fucker" though to outsiders it's less strong, and now in modern black culture the original meaning has now been lost.