Eh, I think that's just your mom being obtuse. Language is fluid and evolving and its commonly accepted that that word doesn't literally mean that. Did she also go try to find you a warm hat and gloves everytime someone called you cool?
Some slang means different things to different people. Who am I to tell anyone that 'motherfuc*er' doesn't legitimately mean 'a mother fuc*er'? Referring to someone as slow (obtuse isn't very nice, HR!) because they take a word literally seems unnecessary. This is especially true when plenty of literature suggests that 'motherfuc*er' initially meant exactly what it implies... one who fuc*s mothers.
Not everything is so black and white. We can say that 'the world doesn't literally mean that', but do you really know what the term 'motherfuc*er' entails in Brazil? Or Italy? Or Africa? Or Spain? You may have an idea of what it implies in North America, but most of us have no idea how the term is received in China, Japan, or most places outside of North American films and rap music.
I live right on the border of Ontario and Quebec, and I can safely say that certain terms and slang don't translate well. There's no shortage of slang that doesn't translate on both fronts. I'm more inclined to believe that GSP's English is good enough and that he's been exposed to English speaking cultures enough to know what motherfuc*er refers to. But who am I to really say that.
We could do this all day. There are PLENTY of people out there who believe that referring to something as 'gay' is to suggest that it is lame, or that referring to someone as a 'f a g' is to imply that said person is a douche. But you try convincing the gay community of that. Words that mean one thing to us can have very different meanings to others.