Yeah, it assumes a bit of knowledge in the book. Mostly it's in his head that he calls it that, maybe once in conversation with his sister, so it makes sense that it wasn't used in the film (and of course, for the inaccuracy reason!)

I probably won't see it until it's out on DVD (I rent more dvds than I go to the cinema) but I shall try and remember to let you know what I think... It's never directly described in the book either.. it's all in his parents reactions. I knew it was coming in the book (the friend had explained the entire plot of the book to me and yet I still felt the need to read it myself..) and it was still awful!