1. In place of a swear word
2. Describing a topic that is obscene, disgusting, or in general "not for kids"
Origin: Circa 1998
At the snopes.com message board, an obviously irate woman posted saying the other posters should watch their language, because there were British school children reading the message board. From then on, as a joke, the other snopesters started using NFBSK in place of swear words or in the subject lines of topics that might not be suitable for children (or the squeamish).
2. I'll tell you about my surgery, but I must warn you, it might get a little NFBSK.